OpenAPI
In plain terms
A standard specification for describing REST APIs.
Going deeper
OpenAPI defines an API's endpoints, parameters, and responses in a machine-readable format, enabling docs, client generation, and testing. It's the contract for REST.
Under the hood
Drives code generation, mock servers, and validation; formerly known as Swagger Specification.
Examples
API documentation, client SDK generation, contract testing, Swagger UI.
Think of it like...
Like an architectural blueprint everyone builds and inspects the API against.