WSDL Web Services Description Language
XML format for describing SOAP web services. W3C Recommendation 2007. Defines operations, port types, bindings, messages. WSDL → client stubs via codegen.
Pre-OpenAPI era: WSDL was the enterprise standard for API contracts. Strictly typed via XSD (XML Schema). Verbose: a trivial service can give 500 lines of WSDL. Codegen tools: wsimport (Java), wsdl2py (Python), svcutil (.NET) → strong "code from contract" workflow. Modern equivalent: OpenAPI/Swagger for REST, .proto for gRPC. WSDL lives on in banking, insurance, healthcare where the SOAP stack is 10–20 years old.