OpenAPI
Standard for describing REST APIs. The fork from Swagger 2.0 → donated to Linux Foundation 2015 → OpenAPI Specification (OAS). v3.0 (2017), v3.1 (2021, JSON Schema-compatible).
YAML or JSON. Describes: paths, methods, params, request/response schemas, authentication, examples. Drives auto-generated: docs (Swagger UI, Redoc), client SDKs (openapi-generator), server stubs, mock servers (Prism), testing (Schemathesis). Spring (springdoc-openapi), FastAPI (Python, native), NestJS, .NET, Go frameworks all auto-generate OAS from code. Competitors: gRPC + Protobuf (binary, RPC-style), GraphQL (single endpoint), AsyncAPI (for messaging/event-driven).