JP2 is the wavelet-based compressed image format defined by Part 1 of the JPEG 2000 standard. The format syntax is defined in ISO/IEC 15444-1:2004 (E) - ITU-T Rec. T.800 (08/2002 E) 127 - Annex I - JP2 file format syntax. Note that the final standards are not freely available, but the committee drafts are.
See also Wikipedia:JPEG 2000.