1. A homogenous solid formed by a repeating, three-dimensional pattern of atoms, ions, or molecules and having fixed distances between constituent parts: Some ice crystals formed on the outside of Mary's bedroom window, all of which had regular shapes.
2. A mineral, especially a transparent form of quartz, having a crystalline structure, often characterized by external planar faces: Jenny thought that the glimmering and shining crystal on her mother's desk resembled glass or even ice!