If I remember my electronics from several years ago, the term is derived from piezoelectric effect, which describes the phenomenon of a crystal oscillating when an electric voltage is applied to it. The frequency of the oscillation is a function of the geometry and composition of the crystal. Most electronic (and almost all digital) circuits use these arrangements to generate a timebase (clock) signal in view of their stability.
This same principle has been utilized in making tweeters which did not possess much of a frequency range or power output but lent themselves easily to miniaturization (as there is no coil/diaphragm involved) and therefore have become commonplace in alarm clocks, beepers and the like.
