uniaxial

a. [ Uni + axial. ] 1. (Crystallog.) Having but one optic axis, or line of no double refraction. [ 1913 Webster ]

☞ In uniaxial crystals, the optic axis has the direction of the vertical crystallographic axis. All tetragonal and hexagonal crystals are uniaxial. [ 1913 Webster ]

2. (Biol.) Having only one axis; developing along a single line or plane; -- opposed to multiaxial. [ 1913 Webster ]