MJ2 (also known as Motion JPEG 2000) is a video format defined by Part 3 of the JPEG 2000 standard. Note that the final standards are not freely available, but the committee drafts are. MJ2 supports audio as well.
According to the specification MJ2 is one of several specifications with common formatting, including JPEG 2000, MPEG-4, and Quiktime, sharing a common definition for the structure of a file (a sequence of objects, called boxes here and atoms in MP4 and QuickTime), and a common definition of the general structure of an object.
See also Wikipedia:JPEG 2000.
- Motion JPEG 2000 (committee draft).