> ## Documentation Index
> Fetch the complete documentation index at: https://docs.atum.xyz/llms.txt
> Use this file to discover all available pages before exploring further.
# Glossary
> Definitions for Atum Protocol terms used across the documentation.
## A
Agent
Two uses on Atum: (1) software acting for a [principal](#principal) — see [Know Your Agent](/get-started/identity/know-your-agent); (2) a [settlement provider](#settlement-provider)'s software that fulfills payments — see [Operating a settler agent](/settle-payments/operating-a-settler-agent).
Atum ID
Identification system on the Atum network: signed [credentials](#verifiable-credential-vc) attached to a payment or quote so participants can be recognized without Atum assigning you an account number. Atum issues [Known to Atum](#known-to-atum) to participants today. See [What is Atum ID?](/get-started/identity/overview).
Atomic units
The smallest indivisible amount of a token. USDC uses 6 decimals, so `1000000` atomic units = 1.00 USDC.
atum-escrow
The [x402](#x402) payment scheme Atum implements: the payer funds a payment in any supported asset, on any supported chain, and the merchant receives its chosen asset on its chosen destination chain, with Atum converting between them. See [Facilitated payments (x402)](/payment-protocols/x402/overview).
Auction (quote selection)
Off-chain process where settlement operators submit price quotes to fulfill your signed payment request; the Gateway selects one. See [Atum payment lifecycle](/get-started/overview/how-an-atum-payment-works).
## C
CAIP-19
[Chain Agnostic Improvement Proposal 19](https://github.com/ChainAgnostic/CAIPs/blob/main/CAIPs/caip-19.md). Standard string format for identifying a specific token on a specific rail, e.g. `eip155:421614/erc20:0x75faf1…`. See [Corridors and assets](/get-started/concepts/corridors-and-assets).
CAIP-2
[Chain Agnostic Improvement Proposal 2](https://github.com/ChainAgnostic/CAIPs/blob/main/CAIPs/caip-2.md). Rail identifier prefix used in CAIP-19, e.g. `eip155:421614` for Arbitrum Sepolia.
Corridor
A source asset → destination asset pair for a payment.
Credential (identity)
A signed statement about a participant (a [verifiable credential](#verifiable-credential-vc)), presented with a payment or quote. Distinct from a [payment credential](#credential-payment-credential). See [Credentials](/get-started/identity/credentials).
Credential (payment credential)
A self-contained, signed **payment authorization**: it carries the payer's chosen payment terms plus the signature that authorizes the funds, so a resource server can verify and settle it without the payer being online. Both [x402](#x402) and [MPP](#mpp-machine-payments-protocol) send it in an `Authorization: Payment` (or `PAYMENT-SIGNATURE`) header on the retried request. The term comes from the x402 spec's "payment-as-credential" — read it as *signed payment*, not a login/API credential or an [identity credential](#credential-identity).
Credential type
The kind of [identity credential](#credential-identity): *what* was attested, not *who* the participant is — for example [Known to Atum](#known-to-atum), the type Atum issues today. See [Credentials](/get-started/identity/credentials#types-today) and [Policy enforcement](/get-started/concepts/policy-enforcement).
Counterparty
The receiving side of a payment — merchant, wallet, or institution.
## D
Decline reason
Why one [settlement provider](#settlement-provider) did not submit a price quote for a payment. Counted in `decline_summary`, which rides on the payment and quote request status responses whether or not the payment went on to settle. A decline is one provider's answer rather than the payment's outcome: others can still quote, and on a payment that did fail the summary is what says why nobody funded it. See [Decline reasons](/decline-reasons).
Depositor
The account that holds source funds and signs the payment request. Often your customer or program wallet. Same as [source account](#source-account); not the same as the [originator](#originator).
DID (decentralized identifier)
A URI that names a key or a domain, used as the subject or issuer of a [verifiable credential](#verifiable-credential-vc). Wallet accounts on Atum use `did:pkh:…`; Atum's issuer uses `did:web:…`. See [What is Atum ID?](/get-started/identity/overview).
Destination asset
[CAIP-19](#caip-19) identifier for the token the recipient receives on the destination rail.
## E
Escrow
See [*Settlement vault*](#settlement-vault).
EVM
[Ethereum Virtual Machine](https://ethereum.org/en/developers/docs/evm/) — the execution environment shared by Ethereum, Arbitrum, Base, Celo, and other compatible rails.
## F
Facilitator
The party in the [x402](#x402) protocol that verifies a signed payment credential (`/verify`) and settles it (`/settle`) on behalf of a resource server. Atum's facilitator implements the [atum-escrow](#atum-escrow) scheme and is **keyless** — it holds no settlement wallet, so it never takes custody of your funds; instead, the Atum network's settlers execute escrow and fulfillment transactions to settle payments. See [x402 Facilitator API](/api-reference/x402/introduction).
Fulfillment
The settlement provider delivering tokenized currency to the recipient on the destination rail.
FulfillmentProxy
Atum contract on the destination rail that records fulfillment and delivery proof.
Fulfillment confirmation
See [*Settlement receipt*](#settlement-receipt).
Fulfillment lock
A lock a [settlement provider](#settlement-provider) holds over a payment while it executes the fulfillment, so a second worker cannot broadcast a competing transaction for the same payment. Held whenever releasing it could let a second transaction reach the chain, including while a broadcast's outcome is unknown and while a transaction still occupies the nonce.
## G
Gas
Native rail token (ETH, TRX, CELO, etc.) paid to execute transactions on a rail. Settlement providers [hold gas for settlement](/settle-payments/integration-guide#4-fund-your-settler-wallets). Senders need it only for a one-time [token approval](/get-started/concepts/permit2-and-approvals) on EVM and Tron. See [Atum infrastructure](/get-started/architecture/payment-rails-primer).
## H
Holder
The participant who signs a [verifiable presentation](#verifiable-presentation-vp) (the **presentation holder**) — on Atum, the same account that signed the inner payment request or quote. Not the [source account](#source-account) unless those are the same party. See [Verifiable presentations](/get-started/identity/verifiable-presentation).
## I
Idempotent retry
Trying the **same purchase** again (same [purchase identifier](#purchase-identifier)) so Atum continues that payment instead of starting a new one. How you finish a payment that takes longer than \~30 seconds. See [Idempotency](/payment-protocols/idempotency).
Intent (payment intent)
See [*Payment request*](#payment-request).
intentId
In [MPP](#mpp-machine-payments-protocol), the per-purchase identifier the acceptance side stamps into the `402` challenge. The payer derives the payment's identity from it. Usually taken from a value the route already carries. See [Idempotency](/payment-protocols/idempotency).
Issuer (credential)
The party that signed a [verifiable credential](#verifiable-credential-vc). Atum issues [Known to Atum](#known-to-atum) to participants today. Distinct from a [tokenized money](#tokenized-money) issuer.
## K
Know Your Agent (KYA)
Who is answerable when software spends on your behalf. See [Know Your Agent](/get-started/identity/know-your-agent).
Known to Atum
The [credential](#credential-identity) Atum issues to participants today. It means this account is allowed to operate on the network in a named role (payment provider, settlement provider, or Atum-operated service). It is not a compliance statement and not a customer-identity check. See [Credentials](/get-started/identity/credentials).
## M
MPP (Machine Payments Protocol)
[An open, HTTP 402-based protocol](https://mpp.dev/) for machine-to-machine payments, distinct from [x402](#x402). MPP has no facilitator role — each **payment method** (Stripe, Tempo, or Atum's `atum-escrow`) verifies and settles itself, in-process on the resource server. Atum ships `atum-escrow` as a native MPP payment method — see [Direct payments (MPP)](/payment-protocols/mpp/overview), so named because your server settles directly against the Payment Gateway, with no separate service in between.
Multi-rail
A payment that moves tokenized currency from one payment rail to another (e.g. Arbitrum → Tron). Equivalent to "cross-chain" in blockchain terminology.
## N
NO\_QUOTES\_RECEIVED
Error when no settlement operator submitted a price quote within the [quote deadline](#quote-deadline). Atum-side logs call the same condition `QUOTE_DEADLINE_EXPIRED`, which is an internal name and never appears in a response. See [Troubleshooting](/get-started/support/troubleshooting#no_quotes_received).
Nonce reservation
A ledger row a [settlement provider](#settlement-provider) holds while one transaction occupies one nonce on one chain, so two concurrent settlements cannot claim the same slot. Releasing the row frees the slot, not the nonce value: a retry reuses the nonce recorded on the prior attempt. See [Agent error reference](/settle-payments/error-reference).
## O
Originator
Organization (PSP, wallet, platform) that submits payment requests to the Payment Gateway. Distinct from the [source account](#source-account) that funds the payment — they can be different parties. See [Originator vs source account](/get-started/identity/verifiable-presentation#originator-vs-source-account).
## P
Payment authorization
One-time permission granted by the source account for Atum to move tokens on its behalf. Required once per account and token on EVM and Tron rails. Equivalent to enabling direct debit. Implemented via [Permit2](#permit2).
Payment Gateway
Atum's REST API for prepare, submit, and status. Your primary integration surface.
Payment hold
See [*Settlement vault*](#settlement-vault).
payment-identifier
x402 v2 extension that carries a client-supplied [purchase identifier](#purchase-identifier) inside the payment. The **resource server** declares it as required in its `402`; the payer supplies the id in `PaymentPayload.extensions`. Atum's facilitator refuses a payment that isn't anchored to one. See [Idempotency](/payment-protocols/idempotency) and the [payment-identifier extension docs](https://docs.x402.org/extensions/payment-identifier).
Payment rail
An independent network that moves tokenized currency according to its own rules, speeds, and costs. Ethereum, Solana, Tron, Arbitrum, Base, and Celo are each separate payment rails — the same way ACH, SWIFT, and FedWire are separate rails for traditional money.
Payment request
The signed object you create and submit to start a payment. Describes source account, destination account, amount, and authorization. Submitted to `POST /v1/payments`.
PaymentSubmitter
In [MPP](#mpp-machine-payments-protocol), the server-side seam that settles a verified `atum-escrow` payment. Your server verifies the credential in-process, then hands the payment to a PaymentSubmitter that submits it to the [Payment Gateway](#payment-gateway). It is MPP's in-process equivalent of the [x402](#x402) [facilitator's](#facilitator) settle step — there is no separate facilitator service. See [Accept payments (MPP)](/payment-protocols/mpp/accept-payments/overview).
Permit2
[Smart contract by Uniswap](https://github.com/Uniswap/permit2) used on EVM and Tron source rails to implement payment authorization. Requires a one-time `approve` per account and token. See [Token approvals](/get-started/concepts/permit2-and-approvals).
Phantom broadcast
An RPC node returning success for a transaction it did not actually accept or propagate. Transaction hashes are computed client-side, so a 200 OK is not proof. A [settlement provider](#settlement-provider) detects one by polling the node for the transaction it just sent. See [Agent error reference](/settle-payments/error-reference).
Policy enforcement
Two levers that constrain who may fulfill a payment: an identity list the originator sets on the [payment request](#payment-request), and credential-type / issuer trust Atum applies for the environment. See [Policy enforcement](/get-started/concepts/policy-enforcement).
Price quote (quote)
The fee a settlement operator offers to fulfill a payment on a given corridor. The Gateway selects one quote per payment. See [Auction (quote selection)](#auction-quote-selection).
Principal
The person or company that grants an [agent](#agent) authority to act. See [Know Your Agent](/get-started/identity/know-your-agent).
production-testnet
Atum's hosted **testnet** environment (e.g. `payment-gw.production-testnet.atum.xyz`), the default real-settlement target in the [examples](https://github.com/Atum-Labs/examples). Despite the name, it moves testnet funds only and is not yet hardened for reliability. See [Choose an environment](/payment-protocols/environments).
PSP (payment service provider)
Company that offers payment products to merchants or consumers; a common Atum integrator profile.
Purchase identifier
The stable identity for **one payment**. Reused on every attempt at that payment; never across two genuine purchases. In [x402](#x402) the payer-side carries it (`paymentIdentifier` / [`payment-identifier`](#payment-identifier) extension); in [MPP](#mpp-machine-payments-protocol) the acceptance side stamps it as [`intentId`](#intentid). Derives the gateway's [`request_id`](#request_id). See [Idempotency](/payment-protocols/idempotency).
## Q
Quote deadline
The time at which the winning quote for your payment will be selected. Quotes from settlement operators must arrive by this time. If none arrives in time, the payment fails with [NO\_QUOTES\_RECEIVED](#no_quotes_received).
## R
Recipient
Address credited on the destination rail.
Reconciler
The part of a [settlement provider](#settlement-provider) that watches the chain to resolve a transaction whose outcome is unknown, rather than retrying it blind. A broadcast the agent cannot classify is handed to the reconciler with its nonce and [fulfillment lock](#fulfillment-lock) still held.
request\_id
The client-supplied idempotency key the gateway deduplicates on, scoped to your source account and signing identity. On the [Payment Gateway SDK](/get-started/start-building/build-with-the-sdk) or direct API calls, you set it yourself. On [x402](#x402) and [MPP](#mpp-machine-payments-protocol), it's derived from the [purchase identifier](#purchase-identifier) and your source account. Reusing it resumes the same payment; a new one is a second payment; reusing it with different terms is refused with `409`, not merged. See [Idempotency](/payment-protocols/idempotency).
Resource server
The server that gates an HTTP route behind payment in the [x402](#x402) protocol — issues the `402 Payment Required` challenge and calls the facilitator's `/verify` and `/settle`. Also called the merchant. See [Accept payments](/payment-protocols/x402/accept-payments/overview).
Retry classification
The field on an error response saying what to do about it, drawn from a closed set: retry after a delay, fix the request, use a new identifier, poll, reconcile first, already done, or contact support. Branch on this rather than on the message text. It is absent on the asynchronous `/status` error object, which carries only `code` and `message`, so look the code up on [Error codes](/errors) there. See [Error codes](/errors).
## S
Settlement receipt
Confirmation returned when a payment completes: `payment_id` plus on-chain confirmation references from the source and destination rails. Verifiable records that exist independently of Atum, equivalent to an ACH trace ID or SWIFT UETR.
settlement\_pending
x402 settle outcome meaning the payment was accepted and is still settling. The payer should [re-attempt the same purchase](/payment-protocols/idempotency). Distinct from `settlement_failed` (terminal) and from a refusal (nothing charged).
SettlementPendingError
MPP error raised by `verify()` when the payment was accepted and is still settling. Carries the gateway `paymentId`. The payer should [re-attempt the same purchase](/payment-protocols/idempotency). Distinct from `SettlementFailedError` (terminal) and `PaymentRejectedError` (nothing charged).
Settlement vault
Smart contract on the source rail that locks the depositor's funds until delivery is proven on the destination rail. Funds are only released after confirmed delivery — neither Atum nor the settlement provider can access them without completing the transfer. Also referred to as "escrow" in technical contexts.
Settlement operator
See [*Settlement provider*](#settlement-provider).
Settlement provider
Independent liquidity provider that submits price quotes for and executes payment routes across rails. You do not integrate with settlement providers directly. Also called a settler or settlement operator.
Settler
See [*Settlement provider*](#settlement-provider).
Settler requirements
Optional allow and deny lists of [settlement provider](#settlement-provider) identities on a [payment request](#payment-request). The originator sets them per payment; Atum applies them. They match named identities, not credential types. See [Payment request](/get-started/concepts/payment-intent#settlement-providers-on-this-payment) and [Policy enforcement](/get-started/concepts/policy-enforcement).
Source account
The wallet that funds the payment. Often called the [depositor](#depositor). Distinct from the [originator](#originator) who submits the request.
Source asset
[CAIP-19](#caip-19) identifier for the token the depositor pays from on the source rail.
Stablecoin
See [*Tokenized money*](#tokenized-money).
## T
Tokenized money
A digital representation of fiat currency that lives on a payment rail. USDC (issued by [Circle](https://www.circle.com/usdc)) and USDT (issued by [Tether](https://tether.to/)) are dollar-denominated tokenized money — always worth \$1, always redeemable. Sometimes called a stablecoin.
Tempo
[Stripe's L1 blockchain](https://tempo.xyz/) for agentic commerce. Supported by Atum. Notably, Tempo has **no native gas token** — transaction fees are paid in USD-denominated tokenized currencies (e.g. pathUSD). See [Tempo assets](/get-started/reference/supported-assets#tempo-assets) for mainnet identifiers and [Tempo Moderato assets](/get-started/reference/supported-assets#tempo-moderato-assets) for the testnet list and the gas note.
Timeline
`GET /v1/payments/{id}/timeline` — ordered settlement events for debugging.
TVM
[TRON Virtual Machine](https://developers.tron.network/docs/tvm) — the execution environment for smart contracts on Tron. Atum's Tron contracts are the TVM counterpart to the [EVM](#evm) settlement contracts. See the [EVM and TVM security review (PDF)](https://static.atum.xyz/docs.atum.xyz/security-reviews/cantina_evm_tvm.pdf).
## V
Verifiable credential (VC)
[W3C](https://www.w3.org/TR/vc-data-model-1.1/#credentials) signed JSON document in which an [issuer](#issuer-credential) asserts a fact about a subject identifier. On Atum, presented inside a [verifiable presentation](#verifiable-presentation-vp). See [Credentials](/get-started/identity/credentials).
Verifiable presentation (VP)
[W3C](https://www.w3.org/TR/vc-data-model-1.1/#presentations) signed wrapper that presents one or more [verifiable credentials](#verifiable-credential-vc) for a specific payment in a specific environment. The [holder](#holder) signs it. See [Verifiable presentations](/get-started/identity/verifiable-presentation).
## X
x402
[HTTP 402-based machine payment protocol](https://x402.org). A resource server answers an unpaid request with `402 Payment Required`; a client signs one of the offered payment options and retries; a [facilitator](#facilitator) verifies and settles it. Atum offers a native x402 v2 facilitator for the [atum-escrow](#atum-escrow) scheme (see [Facilitated payments (x402)](/payment-protocols/x402/overview)), so named because the facilitator — not your server — verifies and settles the payment. x402 v2 uses the `PAYMENT-SIGNATURE` (client → server), `PAYMENT-REQUIRED` (server → client, on the `402`), and `PAYMENT-RESPONSE` (server → client, on settlement) headers — not the deprecated v1 `X-PAYMENT` / `X-PAYMENT-RESPONSE` headers.
## Next steps
* [Atum infrastructure](/get-started/architecture/payment-rails-primer)
* [Core concepts](/get-started/architecture/core-concepts)
* [What is Atum ID?](/get-started/identity/overview)
* [Know Your Agent](/get-started/identity/know-your-agent)
* [Supported networks](/get-started/reference/supported-networks)
* [Facilitated payments (x402)](/payment-protocols/x402/overview)
* [Direct payments (MPP)](/payment-protocols/mpp/overview)