The resolution or splitting of a light wave into two unequally reflected or transmitted waves by an optically anisotropic medium such as calcite or quartz. Also called double refraction. [Answers.com]
Crystalline materials may have different refractive index (n) associated with different crystallographic directions. A common situation with mineral crystals is that there are two distinct n, and they are called birefringent materials. If the y- and z- directions are equivalent in terms of the crystalline forces, then the x-axis is unique and is called the optic axis of the material.
The propagation of light along the optic axis would be independent of its polarization; it’s electric field is everywhere perpendicular to the optic axis and it is called the ordinary- or o-wave.
The light wave with E-field parallel to the optic axis is called the extraordinary- or e-wave. Birefringent materials are used widely in optics to produce polarising prisms and retarder plates such as the quater wave plate. Putting a birefringent material between crossed polarisers can give rise to interference colours.
A widely used birefringent material is calcite. Its birefringence is extremely large, with indices of refraction for the o- and e-rays of 1.6584 and 1.4864 respectively. [http://hyperphysics.phy-astr.gsu.edu]
Optics 