The most common API definition languages we spot in the wild are Swagger / OpenAPI Spec, RAML and API Blueprint. All three let you define your endpoints, your resources, your query or path parameters, your headers, status codes, security schemes, and more.
In a nutshell, these definition languages define the structure of your API, and allow you to describe many elements. As standards in the API industry evolve, however, their purpose and design are under continuous scrutiny.
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