The Collaborative International Dictionary of English v.0.48:

Uniaxial \U`ni*ax"i*al\, a. [Uni + axial.]
   1. (Crystallog.) Having but one optic axis, or line of no
      double refraction.
      [1913 Webster]

   Note: 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]