• Manufacturers' Junction Railway's reporting mark
• Líneas Aéreas Privadas Argentinas' IATA code
• Jeep Comanche
  Jeep Comanche
  The Jeep Comanche is a pickup truck version of the Cherokee compact SUV that was produced from 1985 to 1992. Rear wheel and four-wheel drive models were available, as well as two cargo box lengths of six-feet or seven-feet .-Introduction:During the mid-1980s, according to AMC Chairman W...
  
   or MJ, a pickup truck
• Millijoule
  Joule
  The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
  
   (mJ) or megajoule (MJ), units of energy
• MJ, a defunct New York City subway service

People

• Michael Jackson
  Michael Jackson
  Michael Joseph Jackson was an American recording artist, entertainer, and businessman. Referred to as the King of Pop, or by his initials MJ, Jackson is recognized as the most successful entertainer of all time by Guinness World Records...
  
  , American recording artist, entertainer, and businessman
• Michael Jordan
  Michael Jordan
  Michael Jeffrey Jordan is a former American professional basketball player, active entrepreneur, and majority owner of the Charlotte Bobcats...
  
  , former American professional basketball player

Fictional

• M.J. Delfino, Mike and Susan Delfino's son on Desperate Housewives
• Mary Jane Watson
  Mary Jane Watson
  Mary Jane Watson, often shortened to MJ, is a fictional supporting character appearing, originally, in Marvel comic books and, later, in multiple spin-offs and dramatizations of the Spider-Man titles as the best friend, love interest, and one-time wife of Peter Parker, the alter ego of Spider-Man...
  
  , a character in Marvel's Spider-Man comic book series
• Agent MJ, a character in the movie Miss Cast Away and the Island Girls