Hysteresis occurs in ferromagnetic materials and ferroelectric materials, as well as in the deformation of some materials in response to a varying force.
In natural systems hysteresis is often associated with irreversible thermodynamic change.
When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the imposed magnetizing field is removed. It must be driven back to zero by a field in the opposite direction. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop.