OpenAPI, also known as the Open API Initiative, is an open-source software specification for defining and documenting a RESTful API. It consists of a set of guidelines for developing and consuming machine-readable API descriptions. It provides an easy way to create, maintain and consume RESTful APIs.

An OpenAPI document is a JSON or YAML formatted text file that describes the structure of a RESTful web API service. It contains information about the API's environment, such as the server address, API paths, input/output parameters, authentication requirements and more. It also contains service-specific information, such as the API's available endpoints and operations, as well as a list of operations.

OpenAPI is used in many API-related services, such as API management tools, API gateways, API monitoring tools and API testing tools. It helps to promote API standardization and interoperability, as well as make API consumption easier to understand.

- API Documentation: OpenAPI documents provide a comprehensive reference to API endpoints, their parameters and their description, offering a common language to describe API operations.

- API Governance: OpenAPI documents can be used to validate API requests, ensuring compliance with applicable guidelines.

- SDK Generation: Generating a library of code to access an API, such as an SDK, can easily be done with OpenAPI.

- API Monitoring: OpenAPI documents provide a way to not just monitor API requests, but also to monitor the API response times and errors.