When referencing games in general, Unit refers to the amount of a particular game or console that was sold, I.E. 500,000 units of the N64 were sold on its opening day.
This term is also used in strategy games to denote a person, machine, or other singular entity, such as a soldier. Examples include an Archon in StarCraft and a Terraformer in Sid Meier's Alpha Centauri.