1. Crystal is a natural or synthetic piezoelectric or semiconductor material whose atoms are arranged with some degree of geometric regularity.
2. It is a material distinguished by the arrangement of its atoms into a redundant pattern called a lattice that presents, in fragments of various sizes, a characteristic polyhedral shape. Common shapes include cubes, parallelepipeds, and hexagonal prisms.
3. It is a fragment of material as defined in (2).
4. It is a plate or bar cut from a piece of piezoelectric material.