Answer: MPEG AudioLayer III

Two decades after its public release, MP3 audio files dominate the digital music distribution networks. The highly compressed audio format makes it easy to shuttle music files across the internet, stream audio, and pack your stand-alone music player or smartphone with hours of tunes–with room to spare.

But what exactly does that MP3 extension stand for? MP3s come in two official flavors, MPEG-1 Audio Layer III and MPEG-2 Audio Layer III–if you downloaded any MP3s in the mid-1990s you definitely grabbed the MPEG-1 version, by the late 1990s MPEG-2 had replaced the prior version. The MPEG in the name stands for Moving Picture Experts Group, a group of experts that set the standards for audio and video compression and transmission algorithms.

That explains the MP in the extension and name convention, by what about the 3? The MPEG format has 3 specific audio layers I, II, and III. Each layer refers to a specific type of audio encoding that the MPEG format is capable of handling. Layer III is simply the best suited to high-quality but high-compression audio encoding necessary to create small and easily transferable bundles of audio.