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.
Imgur uses an MP4 video format for converting animated GIFs uploaded by users into its own "GIFV" format, which is actually an MPR referenced in a video element of an HTML 5 document, which is served with a .gifv extension from their servers.
- ISO/IEC 14496-14:2003 (MP4 file format) (not free to download)