Any material which provides a conversion between mechanical and electrical energy.
Piezo is a Greek term which means "to squeeze".
For a piezoelectric crystal, if mechanical stresses are applied on two opposite faces, electrical charges appear on some other pair of faces. Conversely, when an electrical voltage is applied across a piezoelectric material, the material deforms. The piezoelectric property of materials is used in transducers that convert acceleration (or force) into electrical signals, and vice versa.
Piezoelectric materials exhibit the Perovskite crystal structure.
The temperature at which the crystalline structure changes from a piezoelectric to a non-piezoelectric, form. At this temperature PZT (Plumbum "lead" Zirconate Titanate) ceramics lose their piezoelectric properties.