1) Double refraction 2) Refraction 
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material....
Is the maximum numerical difference between the two refractive indices of an anisotropic material.
The strength of double refraction measured by taking the difference between the high and low refractive indices of a doubly refractive gemstone.
when a single incident beam is split into two refracted beams. Both of the refracted beams are parallel, with one offset from the other.
for an anisotropie medium characterized by two refractive indices in the same direction the existence of different velocities of propagation for different, orthogonal polarizations
<chemistry> Optical property of a material in which the refractive index is different for light polarized in one plane compared to the orthogonal plane. ... May arise from molecular organisation of the material (form birefringence.), alignment of molecules due to tension (stress birefringence.) or alignment of rod like particles in flow (flow...
(bi″re-frin´jәns) the quality of transmitting light unequally in different directions.
The numerical measurement of double refraction ingemstones.
Another name for double refraction where the light entering the stone is split into two light rays, and the rays travel in different paths. 
The appearence under the microscope of congo red staining of amyloid.
Optical property of a material in which the refractive index is different for light polarized in one plane compared to the orthogonal plane. See birefringent.
Type: Term Pronunciation: bī′rē-frin′jens Synonyms: double refraction
In a anisotropic or doubly refractive mineral, birefringence is the difference in values between the highest refractive indices and lowest refractive indices; causing incident light to be split into two rays.