Coercivity is the strength of the magnetic field required to demagnetise a ferromagnetic material.  It is also described as the ability of a material to resist demagnetization.

Materials with high coercivity are made into permanent magnets, such as Alnico.  The unit of coercivity is ampere/meter.

Materials with low coercivity are made into electromagnets, such as soft iron.

The coercivity can be calculated from the B-H curve of a material.  The horizontal distance between points b and a in the BH curve in the right is the coercivity

The unit of coercivity is Ampere/metre