The MIME type audio/mpeg3 is used to denote the presence of MPEG-1 Audio Layer 3 audio files. This MIME type falls under the category "audio".

The MPEG-1 Audio Layer 3 format audio files carry the extension MP3. The extension MP3 is also the term that is commonly used to refer to this format. It is a patented digital audio encoding format that uses a type of lossy data compression. MP3 is the standard format used for consumer audio storage as well as for audio playback in digital audio players. The Moving Pictures Experts Group (MPEG) created MP3 as an audio specific format as part of the MPEG-1 standard. In 1991, the format was standardized as an ISO/IEC standard.

The lossy algorithm used in the MP3 format ensures that the amount of data required to store the audio recording is reduced without losing noticeable quality in the process. The process hinges on using a predefined set of frequencies that are taken out of the spectrum, hence the loss of quality. This is why it is called a lossy format. Certain frequencies are also attenuated to reduce the amount of data used to represent them. All these frequencies are deemed beyond the capacity of the human ear to distinguish.