A definition (from: Web Services Architecture - W3C Working Draft 8 August 2003, http://www.w3.org/TR/ws-arch/)
A Web service is a software system designed to support interoperable machine-to-machine interaction over a network. It 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.
A Web service is an abstract notion that must be implemented by a concrete agent:
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