CARS.COM — RPM stands for revolutions per minute, and it's used as a measure of how fast any machine is operating at a given time. In cars, rpm measures how many times the engine's crankshaft makes one full rotation every minute, and along with it, how many times each piston goes up and down in its cylinder.
Related: What Does 'GT' Mean in Cars?
Car engine rpm increases as you press the accelerator, as does power — at least to a point. An engine doesn't necessarily produce its maximum power at its highest rpm. Engine specifications typically present the peak horsepower figure followed by the rpm at which it occurs, such as 252 hp at 5,600 rpm. Torque, a measure of the engine's instantaneous twisting force, typically comes at lower rpm and may appear as a range in turbocharged or supercharged engines, such as 273 pounds-feet at 1,600-4,500 rpm.
Many cars have a tachometer to indicate engine rpm, usually measured in thousands. At the top of the tachometer range is a zone called the redline — usually highlighted literally, with a red line. Revving the engine beyond redline can cause damage. This is really a concern only for cars equipped with a manual transmission; vehicles with an automatic transmission are programmed to shift before the engine speed reaches that point. That will vary, too, depending on how hard you're pressing the accelerator pedal.
In normal driving, an automatic transmission will shift at whatever engine rpm produces the best combination of efficiency and smoothness, making the tachometer redundant (even if it is fun to watch). Drivers with a manual transmission have to master that skill for themselves, and the tachometer can help with that. In newer cars with manual transmissions, a rev limiter tends to prevent the engine from running into the redline, which eliminates potential damage, but it's up to the driver to recover from the sometimes dramatic interruption and shift to a higher gear.