Definition of "Crystal" :

noun: a protective cover that protects the face of a watch

noun: glassware made of quartz

noun: a crystalline element used as a component in various electronic devices

noun: a rock formed by the solidification of a substance; has regularly repeating internal structure; external plane faces

noun: colorless glass made of almost pure silica

noun: a solid formed by the solidification of a chemical and having a highly regular atomic structure