Revolutions per minute
The amount of revolutions the crankshaft in an internal combustion engine takes in 1 minute. Its the "speed" of the engine. The faster it rotates, generally speaking, the closer you'll get to its maximum torque and horsepower output.
Trust me, it doesn't take one minute for the crankshaft to turn once. If it did, we would get like 500 miles to the gallon.