What does 'RPM' stand for?

RPM stands for revolutions per minute.

It is a measure of rotational speed: the number of complete turns a rotating object makes in one minute. In a car, a tachometer displays the engine’s crankshaft speed in RPM. A reading of 3,000 RPM means the crankshaft is completing 3,000 revolutions every 60 seconds.

RPM is useful because engine speed affects power delivery, fuel consumption, noise, and mechanical stress. Drivers typically use the tachometer to keep the engine within an efficient operating range and to avoid the redline, where excessive speed can damage components. The exact useful range depends on engine design; a small petrol engine and a large diesel engine may have very different operating speeds.

RPM is related to, but not identical with, angular speed. One revolution per minute equals 1/60 of a revolution per second, while 60 RPM equals 1 hertz. In strict SI usage, rotational frequency is expressed in hertz and angular speed in radians per second, but RPM remains standard in automotive instruments and technical discussion. “Rotations per minute” is a common informal phrase, but the established expansion is “revolutions per minute.”