A Web client that is used directly by a person. (An example of a client that is not used directly is the client that fetches pages to be indexed by a search engine.)
Adtitional information in a textual document that conveys semantics or presentation. (Note: while XML is called a markup language, it is often used purely for its tree structure, rather than for markup.)
Web, the
A world-wide system for transmission of information, for which there are a set of standards (de jure and de facto) and many clients (primarily browsers) and servers.

Valid HTML 4.0!

(Back to WWW, David Baron)