Define tachometer?

A tachogenerator for detecting the rotation speed of a rotating machine part comprising: at least one magnetically acting encoder which can rotate with the machine part, said encoder comprising magnetically acting areas subdivided into angle parts spaced a distance apart from each other, said distance between said angle parts defining an angular pitch; wherein the magnetically acting areas are composed of ferromagnetic material or a magnetic pole ring; and wherein during rotation said encoder produces a measurement signal.