(MP3) A digital audio compression algorithm that acheives a compression factor of about twelve while preserving sound quality. It does this by optimising the compression according to the range of sound that people can actually hear. MP3 is currently (July 1999) the most powerful algorithm in a series of audio encoding standards developed under the sponsorship of the Moving Picture Experts Group (MPEG) and formalised by the International Organization for Standardization (ISO).

MP3 is very different from Layer 2, using an additional MDCT layer to increase frequency resolution. Its scale factor groups are more optimised for the human ear, and it uses nonlinear sample quantisation and Huffman coding.

MP3 files (filename extension ".mp3") can be downloaded from many World-Wide Web sites and can be played using software available for most operating systems (also downloadable), e.g. Winamp for PC, MacAmp for Macintosh, and mpeg123 for Unix.

MP3 files are usually downloaded completely before playing but streaming MP3 is also possible. A program called a "ripper" can be used to copy a selection from a music CD onto your hard disk and another program called an encoder can convert it to an MP3 file.