a data compression format for encoding digital audio, most commonly music. MP3 files offer substantial fidelity to compact disc (CD) sources at vastly reduced file sizes.

In 1993 the Moving Picture Experts Group (MPEG) at the International Organization for Standardization (ISO) released the MPEG-1 standard for video and audio compression. MPEG-1 included three schemes, or layers, for audio encoding, of which the third—called MP3—quickly became the most popular through the wide availability of simple computer programs (computer program) for compressing music files.

MP3 encoding reduces the size of a CD audio file by discarding certain sounds based on assumptions of what the ear is least likely to miss. Different levels of compression are available, with higher-fidelity encoding yielding larger files. An MP3 file can be played directly on a personal computer (PC) or portable digital music player, such as Apple Inc.'s iPod, or written onto a standard audio CD, although the data loss from compression is not reversible.

By the early 21st century the average consumer could store millions of songs in the MP3 format on a PC or MP3 player. Online services allowed computer users to share their music files with millions of others. Yet even as musicians and consumers began posting downloadable MP3 files online as a way of directly reaching listeners, recording companies took legal action to prevent the unauthorized distribution of copyrighted recordings. Meanwhile, legitimate Internet e-commerce sites, such as Apple's iTunes Store, sprang up to serve the market, selling individual songs that could be downloaded in a matter of seconds and changing forever the distribution of musical recordings (music recording).