Icecast is a streaming media project, released as free software, maintained by the Xiph.org Foundation. It also refers specifically to the server program which is part of the project. Live audio streams are commonly streamed using either Icecast or SHOUTcast servers.
Icecast, the project, is a collection of programs and libraries for streaming audio over the Internet.
Icecast, the server software, is a streaming media server, which officially supports Opus, Ogg Vorbis and other free codec streams; MP3 and AAC audio streams are known to work too. It also supports video streaming using either WebM or Theora. It can be used to create an Internet radio station or a privately running jukebox and many things in between. It is very versatile in that new formats can be added relatively easily and it supports open standards for communication and interaction.
More information can be found on icecast.org. Also latest documentation is available.