Web Application Description Language (WADL) is an XML-based language used to describe the functionalities of RESTful web services. It provides a machine-readable description of the resources, methods, request, and response messages that a web service supports, making it easier for developers to use and integrate these web services into their applications. WADL is commonly used in conjunction with tools and frameworks such as Apache CXF and Jersey to generate client-side code for consuming web services.