Polarizability is a measure of how easily an electron cloud is distorted by an electric field. Typically the electron cloud will belong to an atom or molecule or ion. The electric field could be caused, for example, by an electrode or a nearby cation or anion.
If an electron cloud is easy to distort, we say that the species it belongs to is polarizable.
The units of α are C m2 V-1.
Large, negatively charged ions, such as I- and Br-, are highly polarizable.
Small ions with high positive charge, such as Mg2+ and Al3+ have low polarizability, but they have a high ability to polarize polarizable species, such as I- and Br-.
In ordinary usage polarizability refers to the "mean polarizability", i.e., the average over the x,y,z axes of the molecule. Polarizabilities in different directions (e.g. along the bond in Cl2, called "longitudinal polarizability", and in the direction perpendicular to the bond, called "transverse polarizability") can be distinguished, at least in principle.
Polarizability along the bond joining a substituent to the rest of the molecule is seen in certain modern theoretical approaches as a factor influencing chemical reactivity, etc., and parametrization thereof has been proposed. See INGOLD (1953); TAFT and TOPSOM (1987).