Icecast is an open source streaming media project maintained by the creators of the Ogg media format, Xiph.org Foundation. It also refers specifically to the server program which is part of the project.
Ices comes in two flavors, the 0.x flavor which supports sending an MP3 stream to an icecast server, and the 2.x flavor which supports sending an Ogg Vorbis stream to an icecast server.
Related: ShoutCast