From The Free On-line Dictionary of Computing (19 Sep 2003) [foldoc]:
IRC
Internet Relay Chat
From Jargon File (4.4.4, 14 Aug 2003) [jargon]:
IRC
/I.R.C/, n.
[Internet Relay Chat] A worldwide "party line" network that allows
one to converse with others in real time. IRC is structured as a
network of Internet servers, each of which accepts connections from
client programs, one per user. The IRC community and the Usenet and
MUD communities overlap to some extent, including both hackers and
regular folks who have discovered the wonders of computer networks.
Some Usenet jargon has been adopted on IRC, as have some conventions
such as emoticons. There is also a vigorous native jargon,
represented in this lexicon by entries marked `[IRC]'. See also {talk
mode}.