MP3 is the popular name for MPEG-1 (and MPEG-2) Audio Layer III, a perceptual audio compression format defined as Layer III of the audio part of the MPEG-1 standard. The specification is ISO/IEC 11172-3, “Information technology - Coding of moving pictures and associated audio for digital storage media at up to about 1,5 Mbit/s - Part 3: Audio,” whose ISO catalogue record shows a first edition dated 1993-08-01. The MPEG-1 audio specification defines three layers of increasing complexity and compression efficiency, with Layer III being the most sophisticated and the one that became a household name.
Note: ISO and ITU standards pages are often paywalled and may not load freely. The ISO catalogue record for 11172-3 is the authoritative primary source for this format; the ITU-T T.81 record is cited here only as a freely loading example of an official ITU/ISO multimedia standards record from the same era and committee family, to confirm the institutional context. The technical content of MP3 itself lives in 11172-3.
The key idea behind MP3 is perceptual coding. Rather than reproducing the audio waveform exactly, the encoder uses a psychoacoustic model of human hearing to identify which parts of the sound the ear cannot perceive, such as quiet tones masked by louder nearby tones, and discards or coarsely quantizes them. The signal is split into frequency subbands and transformed, and bits are allocated dynamically so that audible components are preserved while inaudible ones are thrown away. This is why MP3 is a lossy codec: the decoded audio is not identical to the original, but at moderate bitrates the differences are designed to be imperceptible to most listeners.
The format emerged from a multi-year research effort centered at the Fraunhofer Institute in Germany, working with university and industry partners within the MPEG standardization process. The result was a codec that could compress CD-quality stereo audio by roughly a factor of ten or more while remaining listenable, turning a several-megabyte song into a file small enough to download over the dial-up and early broadband connections of the late 1990s.
That combination of small size and acceptable quality made MP3 the catalyst for the digital music revolution. It enabled peer-to-peer file sharing, portable music players, and eventually digital music stores, while simultaneously triggering years of legal and commercial conflict over music distribution and copyright. Fraunhofer and its partners licensed the patents covering MP3 encoding and decoding for many years; those patents expired in the late 2010s. Though newer codecs such as AAC and Opus surpass it in efficiency, MP3 remains one of the most recognized and consequential audio formats ever standardized, and a defining example of how a compression standard can reshape an entire industry.