M4A (MPEG-4 Audio Layer) is an audio container format used for storage of compressed audio data. It is part of the MPEG-4 multimedia container format (MP4) specification. The M4A file format emerged as a reaction to the inability of certain file management tools to differentiate between audio-only MP4 files and standard MP4 files, containing multiple audio, video and/or subtitle streams combined together. The format gained initial popularity through Apple Inc., who started selling songs in M4A format on their iTunes Store.

The M4A file format is forecast to become the new standard in audio compression. In comparison to its most popular alternative - the MP3 audio file format, the M4A achieves an audibly better sound quality packed in a smaller file size, while being encoded at the same bit rate. This is achieved mostly due to the enhanced compression schemes and codecs that the M4A format uses, such as the Advanced Audio Coding (AAC) encoding scheme for lossy audio compression and the Apple Lossless Audio Codec (ALAC).

As defined by the format standard, M4A files can also contain metadata. However, in practice, metadata, such as chapter markers, images and hyperlinks, is more commonly stored into M4B files. Another popular derivative of the M4A file format is the M4P format. Contrary to M4A, M4P files contain audio streams encrypted using the FairPlay DRM (Digital Rights Management) mechanism. The Apple iTunes Store distributes both M4A and M4P files.