MP4
From Just Solve the File Format Problem
MP4 usually refers to the multimedia file format defined in Part 14 (and to a lesser extent Part 1) of the MPEG-4 standard.
"MP4" could also mean the entire MPEG-4 standard, or some other subset of MPEG-4. This article only covers the Part 14 file format.
MPEG-4 Part 14 is an application of Part 12. The recommended extension for it is ".mp4", though ".m4a" is often used for audio-only files. It is standardized by ISO/IEC 14496-14.
