Any material that has different refractive index according to the plane of polarisation of the light. The effect is to rotate the plane of the refracted light so that, using crossed Nicholl prisms (polarisers set at right angles to give complete extinction), the birefringent material appears bright. The birefringence can arise through anisotropy of structure (form birefringence) or through orientation of molecules either because of mechanical stretching (stress birefringence) or because of alignment in flow (flow birefringence). A classic example, often used to demonstrate the effect, is a hair which, because of the orientation of the keratin, shows form birefringence.
Dictionary of molecular biology. 2004.