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 early days of CGI however, the unit of measurement used to indicate a relative fraction of the available width & height (of the column & row) it occupied in a grid layout, was also called a 'null unit'. Therefore it is possible the term was given to any program assigned to locate and pinpoint the position of another program on the Game Grid, as was the function of 'null units' used in grid graphics.

In the film TRON, Sark called his Lieutenant a "null unit" after Tron, Flynn, and Ram escaped from the Game Grid.

We better [get them], null unit!