Uniaxial

a.
  1. (Crystallog.) Having but one optic axis, or line of no double refraction.
  2. (Biol.) Having only one axis; developing along a single line or plane; -- opposed to multiaxial.

In uniaxial crystals, the optic axis has the direction of the vertical crystallographic axis. All tetragonal and hexagonal crystals are uniaxial.