WADL, which stands for Web Application Description Language, is an XML-based machine-readable description of a web application's capabilities. It is designed to describe HTTP-based web applications in a manner similar to how WSDL (Web Services Description Language) describes SOAP-based web services. The primary purpose of WADL is to allow tools and clients to automatically discover and interact with the resources, methods (like GET, POST, PUT, DELETE), parameters, and media types supported by a RESTful web service. It provides a standardized contract for the service, detailing URIs, request formats, response formats, and error conditions. While WADL was an early attempt to standardize REST API documentation, it has largely been superseded in popularity and adoption by the OpenAPI Specification (OAS), often documented using YAML or JSON formats (like Swagger). However, WADL files remain relevant for describing older or specific Java-based RESTful services, particularly those built using frameworks like JAX-RS.