electric susceptibility
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Push two different materials with the same electric field and one will polarize a lot while the other barely budges. Electric susceptibility is the single number that captures this eagerness: how much polarization you get out of a material for each unit of field you put in. A high susceptibility means a material that polarizes readily; a low one means a stubborn material that hardly responds.
Formally, susceptibility is the ratio of the polarization a material develops to the electric field driving it. It is closely tied to the dielectric constant — in fact the dielectric constant is just one plus the susceptibility — but susceptibility isolates the material's own contribution, the part beyond what empty space already does. Materials with easily nudged electron clouds, or with molecules that swivel to follow the field, have large susceptibilities.
This matters because susceptibility is the cleanest measure of how 'polarizable' a material is, and it links the microscopic tilting of charges to the bulk numbers engineers use. The common confusion is treating susceptibility as a fixed constant; for ordinary materials it is steady, but near a ferroelectric's transition temperature it can shoot up enormously, and it generally depends on frequency and temperature. It describes a response, and responses change with conditions.
As a ferroelectric like barium titanate is cooled toward the temperature where it starts to polarize on its own, its electric susceptibility climbs toward a huge peak — the material becomes almost absurdly easy to polarize right at the brink of forming its built-in order.
Near a ferroelectric transition, electric susceptibility spikes — the material polarizes almost effortlessly.
Susceptibility and dielectric constant carry the same information: the dielectric constant equals one plus the susceptibility, the extra 'one' being the response of empty space itself.