The term REV INDICATOR is a four letter word, TACH, which stands for tachometer. A tachometer is an instrument used to measure the speed of a rotating shaft, such as an engine crankshaft, in revolutions per minute (RPM). It is commonly found in automobiles and other motorized vehicles, and is used to monitor the engine's performance. The REV INDICATOR is a visual representation of the engine's RPM, and is typically located on the dashboard of the vehicle.