\^▐kris-tә-lәn\ adj [ME cristallin, fr. MF & L; MF, fr. L crystallines, fr. Gk krystallinos, fr. krystallos] (15c) A substance (usually solid but can be liquid) in which the atoms or molecules are arranged in a definite pattern that is repeated regularly in three dimensions. Crystals tend to develop forms bounded by definitely oriented plane surfaces that are harmonious with their internal structure. They may belong to any of six crystal systems: cubic, hexagonal, tetragonal, orthorhombic, monoclinic, or triclinic.