In optical crystallography, those anisotropic crystals having one direction of apparent isotropy, i.e., one optic axis, corresponding to the unique direction of axial symmetry in the hexagonal, trigonal, and tetragonal crystal systems. Uniaxial crystals are positive if their extreme refractive index (extraordinary ray) is less than their axial refractive index (ordinary ray), negative if greater. Compare: isotropic; isotropy; anisotropy; biaxial.