# Concepts

URL: /reference/concepts · Id: `page:/reference/concepts`

> Every platform concept a Startup meets when it defines, publishes and serves an API, in one sentence each.

The concepts below are the words every page uses, each defined in one sentence and grouped by the part of an API it belongs to.

| Concept | Part | Definition |
| --- | --- | --- |
| <a id="product"></a>[Product](/define/product) | Platform | What a Startup sells; an API Product is one whose Definition declares operations. |
| <a id="definition"></a>Definition | Platform | The one record every [Interface](/interfaces) of an API is generated from, so no two Interfaces can disagree. |
| <a id="operation"></a>[Operation](/define/operations) | Contract | One callable Verb on a Noun, named by its operationId, with its path, parameters, return schema, example, price and run. |
| <a id="interface"></a>[Interface](/interfaces) | Contract | One way to reach the same Definition, served on the API host itself: the API, MCP, SDK, Docs and App among them. |
| <a id="machine-register"></a>[Machine register](/interfaces#machine-registers) | Contract | A file an agent reads instead of a page: OpenAPI 3.1, llms.txt, a Markdown twin, agents.json or the pricing document. |
| <a id="wrapped-source"></a>[Wrapped Source](/define/sources) | Contract | An outside source an API [Product](/define/product) wraps, declared as one descriptor: its operations, Offer, meters, limits, tenancy and error map. |
| <a id="namespace"></a>Namespace | Address | The unit of isolation an API serves from; a Customer keeps its calls in the namespace of the Startup that uses the API. |
| <a id="host"></a>[Host](/publish/hosting) | Address | The home Domain of the Startup an API belongs to: every Interface of the API answers there, and every link stays there. |
| <a id="key"></a>[Key](/callers/keys) | Caller | The credential a Customer presents on a call; the platform checks it on every call and never stores it. |
| <a id="entitlement"></a>Entitlement | Caller | What a [Key](/callers/keys) may call: the operations an accepted Offer grants, checked on every call through every Interface. |
| <a id="test-twin"></a>[Test twin](/callers/keys#kinds) | Caller | The separate namespace a test or dev Key calls into; its books never post and it never touches live records. |
| <a id="price"></a>Price | Terms | The posted rate of one call that returns a result: the same number in its cost header and on its usage Event. |
| <a id="meter"></a>Meter | Terms | Counts each priced call once, by the token of its usage Event, so a retried request counts once. |
| <a id="limit"></a>[Limit](/callers/keys#checks) | Terms | A cap checked and reserved before a call runs; when the check cannot run, the call does not run. |
| <a id="batch"></a>Batch | Execution | One request that runs 1 to 100 calls as one Action, each item answering exactly what the API answers alone. |
| <a id="action"></a>Action | Execution | The record a call is accepted as before it runs, so its [Limits](/callers/keys#checks) are reserved and its end is kept. |
| <a id="problem"></a>[Problem](/evidence/errors) | Evidence | The error document a failed call answers: the RFC 9457 fields plus a stable machine code. |
| <a id="call-record"></a>[Call record](/evidence/call-record) | Evidence | The kept record of one call: what was called, by which Key, through which Interface, what it answered and cost. |
| <a id="provenance"></a>[Provenance](/evidence/provenance) | Evidence | Who asserted a fact and how, on four axes; an ingested record that cannot be stamped is rejected, never defaulted. |
| <a id="dashboard"></a>Dashboard | Evidence | The page every keyed API serves at `/dashboard`: one Key and its calls, in HTML, Markdown and JSON. |