- (metrology) Symbol for the megajoule, an SI unit of work or energy equal to 106 joules.
- (slang) Initialism of Mary Jane; marijuana.
- Initialism of mahjong.
2006, Judy Yung, Gordon H. Chang, H. Mark Lai, Chinese American voices: from the gold rush to the present ..., ↑ISBN Invalid ISBN, page 283:
- Then when my sister was old enough, my mother taught the three of us how to play MJ [mah-jongg].