Having but one optical axis, or axis of double refraction. Iceland spar is a uniaxial crystal. See refraction, and cut under interference.In biology, having one main axis to which the other axes are subordinate; growing lengthwise.In botany, having a single axis, as when the primary stem of a plant does not branch and terminates in a flower.Monaxon, as a sponge-spicule.n. A uniaxial crystal.n. Also uniaxal.