The electro-optic effect (or electrooptic effect) is the modification of the refractive index of a medium, caused by an electric field. (Some authors more generally consider electro-optic effects such that any optical properties may be affected by an electric field.) Only non-centrosymmetric materials (mostly nonlinear crystal materials) exhibit the linear electro-optic effect, also called the Pockels effect, where the refractive index change is proportional to the electric field strength (see the article on Pockels effect for more details).
Materials exhibiting the Pockels effect are called electro-optic materials. Some examples for such materials are lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium titanyl phosphate (KTP) and β-barium borate (BBO).
All centrosymmetric media exhibit only the Kerr electro-optic effect, where the refractive index change is proportional to the square of the electric field strength, and is typically much weaker than for the linear effect within the range of electric field strength which the material can tolerate.