A device used to measure speed of rotation.
Even though one purpose of having a tachometer is to help drivers keep their engine speeds down, most of us occasionally try to see how high we can make the needle go.
A tachometer is literally a "speed-measurer," since the Greek root tach- means "speed." This is clear in the name of the tachyon, a particle of matter that travels faster than the speed of light. If it exists, it is so fast that it is impossible to see. Tachycardia is a medical condition in which the heart races uncontrollably. Since the speed that a tachometer measures is speed of rotation, the numbers it reports are usually revolutions per minute, or rpm's.