Null unit is a derogatory slur used by programs in the computer world. While its exact meaning in TRON is unclear, it seems to be used in a similar context as the word "idiot". Null is an English word meaning "nothing" or "valueless", and in general computing terms, "null" usually refers to something indicating "no value" or an "unknown value". In the film TRON, Sark called his Lieutenant a "null unit" after Tron, Flynn, and Ram escaped from the Game Grid.