Polarization is the distortion of the charge distribution about an ion or molecule. In effect, the electron cloud about, for example, an anion can become distorted by a neighboring cation, thereby affecting their determined ionic radii. The anion, now polarized by the cation, shares its electrons with the cation and covalent character increases. Polarization may be temporary if the electric field is temporary, and thus a temporary dipole moment may form.
Cf., polarizability