HJ may refer to:
Science, technology, and mathematics Hall–Janko group, a mathematical group Hamilton–Jacobi equation, an equation in classical mechanics which also has relations to quantum physics U.S. code for a cryptographic key change; see cryptoperiod
Other uses , a two-letter combination used in some languages hj-reduction in English, dropping the sound before Hajji (Hj.), an Islamic honorific Handjob hic jacet ('here lies'), Latin phrase on gravestones Hilal-i-Jurat, post-nominal for Pakistan honour Hitler-Jugend (Hitler Youth) Holden HJ, an Australian car 1974-1976 Hot Jupiter, a type of planet Tasman Cargo Airlines, IATA airline designator Jiaguwen Heji, the standard comprehensive collection of rubbings of ancient Chinese oracle bone inscriptions "High jump" athletics abbreviation in track and field