(Or "chat room", "room", depending on the system in question) The basic unit of group discussion in chat systems like IRC. Once one joins a channel, everything one types is read by others on that channel. Channels can either be named with numbers or with strings that begin with a "#" sign and can have topic descriptions (which are generally irrelevant to the actual subject of discussion).
Some notable channels are "#initgame", "#hottub" and "#report". At times of international crisis, "#report" has hundreds of members, some of whom take turns listening to various news services and typing in summaries of the news, or in some cases, giving first-hand accounts of the action (e.g. Scud missile attacks in Tel Aviv during the Gulf War in 1991).