The domain name that is reserved for example purposes, along with example.org and example.net. They're defined specifically for this purpose in Section 3 of RFC 2606. The domains themselves are maintained by IANA.
This is just about the only safe domain name you can refer to if you want a nonexistent domain for purposes of discussion; every other domain, no matter how ridiculous it is, might get registered in "real life". Thus, it's used in all Internet-related documentation (in RFCs, for example) as the example domain.