Illustrated Glossary of Organic Chemistry

Polarizability: The ability of a bond or molecule to be polarized by distorting its electron cloud.

Before polarization:
Electron density evenly distributed
After polarization:
Electron density unevenly distributed
Distortion of a spherical electron cloud. When polarization occurs easily, the species is said to be soft. When polarization occurs only with difficulty the species is said to be hard.