A biaxial nematic is a spatially homogeneous liquid crystal with three distinct optical axes. This is to be contrasted to a simple nematic, which has a single preferred axis, around which the system is rotationally symmetric. The symmetry group of a biaxial nematic is D_2h i.e. that of a rectangular right parallelepiped, having 3 orthogonal C₂ axes and three orthogonal mirror planes. In a frame co-aligned with optical axes the second rank order parameter tensor of a biaxial nematic has the form

where

S is the standard nematic scalar order parameter

T a measure of the biaxiality.

The first report of a biaxial nematic appeared in 2004 [Phys. Rev. Lett., 92, 145505 (2004)].