As Webster 1913 would probably say.

The ability to be polarized.

In chemistry, polarizability refers to the property of a substance to be polarized by an electric field. Under a strong field, the electrons on an atom or molecule may shift their relative density such that they align more in the field direction (like a weather vane aligning with the wind). As a result, all the dipoles aligning cause an increase in the local electric field.

Polarizability is empirically defined as a constant that relates dipole moment to external electric field:

p = k E

Where p is the dipole moment, E is the field, and k is the polarizability.

This quantity is also related to the dielectric constant of a medium. Polarizability affects the strength of electrostatic interactions through space. If the environment has low polarizability (the lowest being vacuum, as it has no charges to move), then Coulomb's Law shows those electrostatic interactions will be the strongest. In highly polarizable media such as water, charges can be screened by local dipole reorientation, resulting in weaker through-space interactions.

Capacitors are designed to have dielectrics with very low polarizability, in order to prevent conduction of charge from one plate to another. Dielectrics in capacitors also have the advantage that slight polarizability allows a greater accumulation of charge on the opposing plates, increasing the capacitance of the component.