Any rapid, repeating undulatory or tremulous movement; it is vibration that causes all sound.<br><br>A force which oscillates about some specified reference point. Vibration is commonly expressed in terms of frequency such as cycles per second (cps), Hertz (Hz), cycles per Minute (cpm) or (rpm) and Strokes per Minute (spm). This is the number of oscillations which occurs in that time period. The amplitude is the magnitude or distance of travel of the force.<br><br>The rapid back-and-fourth movement, often invisible, in space of an object against which some force has been applied.