From AMS Glossary
The proportionality factor between the dipole moment (usually electric but also magnetic) induced in an atom, molecule, or even a particle and the inducing electric or magnetic field.
If the induced electric dipole moment is p,
where E is the electric field and α the electric polarizability. In general, α is a tensor (because p and E are not necessarily parallel) and depends on the frequency of the (periodic) field E.