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A software component or system designed to support interoperable machine- or application- oriented interaction over a network. A Web service has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the Web service in a manner prescribed by its description using SOAP messages, typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards.
Source(s):
NIST SP 800-95
Open Grid Services Architecture Glossary of Terms (25 January 2005)