Abbreviation for the first person shooter Rise Of The Triad
. A very sweet game, given the technology of the time (just after Doom). Unlike Doom, in ROTT, you could have "rooms over rooms", albeit in a very limited fashion.
If Wolfenstein 3D was really 2D, Doom was 2.5D, and Quake was 3D, ROTT probably weighs in at 2.75D.