HJ

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

 * $⟨hj⟩$, 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