FLAC

Short for Free Lossless Audio Codec, FLAC is an open source audio codec that has little compression and has little audio loss, meaning the FLAC files are much larger than a lossy file such as an MP3 file but will sound identical to the source they were copied from.

The official FLAC page is found at http://flac.sourceforge.net/ 

Also see: Codec, Lossless, Sound definitions