1. a. A homogenous solid formed by a repeating, three-dimensional pattern of atoms, ions, or molecules and having fixed distances between constituent parts. b. The unit cell of such a pattern.

2. A mineral, especially a transparent form of quartz, having a crystalline structure, often characterized by external planar faces.

3. a. A natural or synthetic crystalline material having piezoelectric or semiconducting properties. b. An electronic device, such as an oscillator or a detector, using such a material.

4. a. A high-quality clear, colorless glass. b. An object, especially a vessel or an ornament, made of such glass. c. Such objects considered as a group.

5. A clear glass or plastic protective cover for the face of a watch or clock.

6. Slang. A stimulant drug, usually methamphetamine, in its powdered form.