The property of certain crystals that causes them to produce voltage when a mechanical pressure is applied to them such as sound vibrations. This technique is used to build crystal microphones, phonograph cartridges and strain gauges, all of which turn mechanical movement into voltage.
The piezoelectric effect also works in reverse, causing an oscillation when the material is subjected to electrical pressure (voltage). For example, earphones and inkjet nozzles are made with piezoelectric materials. See inkjet printer