Ok, from the systems integration view of web services, my preference is to take the W3C Web Services Requirements definition (being the World Wide WEB consortium) which is:

“A web service is a software application identified by a URI, whose interfaces and binding are capable of being defined, described and discovered by XML artefacts and supports direct interactions with other software applications using XML based messages via Internet-based protocols.”

Infact, even the W3C give differing definitions (W3C Web Services Architecture Group)… but I believe the above definition is flexible enough to cater for different technologies and transport protocols.