1. The production of electricity or electric polarity by applying a mechanical stress to certain crystals.
2. The generation of an electric charge in certain nonconducting materials, such as quartz crystals and ceramics, when they are subjected to mechanical stress including pressure or vibration, or the generation of vibrations in such materials when they are subjected to an electric field.

Piezoelectric materials exposed to a fairly constant electric field tend to vibrate at a precise frequency with very little variation, making them useful as time-keeping devices in electronic clocks, as used in wristwatches, and computers.