The OpenAPI Specification (OAS) is a standard format for describing RESTful APIs. It allows developers to define the structure of their API, including endpoints, request parameters, and response formats, in a machine-readable way. This makes it easier for both humans and computers to understand how to interact with the API. By using the OpenAPI Specification, teams can generate documentation automatically, create client libraries, and even test their APIs more efficiently. It promotes consistency and helps ensure that everyone involved in the development process has a clear understanding of how the API works, leading to better collaboration and fewer errors.