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A standard way to describe an API so that tools can automatically generate documentation and client code — like a blueprint that both the architect and the builder can read.
A specification (formerly Swagger) for describing RESTful APIs in a machine-readable YAML or JSON format. An OpenAPI document defines endpoints, request/response schemas, authentication methods, and error codes, enabling auto-generated docs, client SDKs, and mock servers.
OpenAPI 3.1 aligns its Schema Object with JSON Schema 2020-12, supporting $ref, discriminator, oneOf/anyOf, and content media type encoding. The specification defines a document structure (info, servers, paths, components, security) that toolchains (Swagger UI, Redoc, openapi-generator) consume for documentation, code generation, and contract testing. Semantic versioning of the API document tracks breaking changes, and overlay documents (OAS 3.1) allow non-destructive patching of generated specs.