It's in the first line of the REST chapter[1] of the original thesis:

    (...) Representational State Transfer (REST) architectural style for distributed
    hypermedia systems (...)

Roy Fielding also talks about Hypertext[2], which means essentially the same (hypermedia being an expansion of the concept of hypertext).

[1]: http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch...

[2]: http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hyperte...