Original: Colloquy is an open-source IRC, SILC, ICB and XMPP client for Mac OS X. Colloquy uses its own core, known as Chat Core, although in the past it used Irssi as its IRC protocol engine. One of the primary goals behind Colloquy was to create an IRC, SILC and ICB client with Mac OS X visuals.
Simplified: Colloquy is an IRC client that first came out in 2005.