A channel is a named group of one or more clients which will all receive messages addressed to that channel.
– J. Oikarinen, D. Reed; Internet Relay Chat Protocol; RFC 1459; May 1993.
An IRC channel is a text-based, virtual meeting room, generically called chat rooms”. Each channel is a virtual space where anybody connected to the Internet and IRC can join by entering and talking to the other people already there.
You can leave a channel at any time, join another channel, and rejoin channels whenever you wish. Many IRC applications will let you join more than one channel at the same time.