Retentivity:
Retentivity refers to the ability of a material to retain its magnetization after the removal of an external magnetic field. It is the measure of the residual magnetic induction left behind in a material when the magnetizing field is removed.

Explanation:
When a ferromagnetic material is subjected to an external magnetic field, the alignment of its atomic dipoles changes, resulting in the material becoming magnetized. This magnetization process is called the magnetization curve.

During the magnetization process, the magnetic domains within the material align themselves with the external magnetic field. These domains consist of groups of atoms that have their magnetic moments aligned in a particular direction. When the external magnetic field is applied, the domains align themselves in the direction of the field, resulting in magnetization.

Hysteresis:
Hysteresis is the phenomenon where the magnetic induction in a material lags behind the change in the magnetizing field. In other words, it is the lagging of the magnetic properties of a material behind the changes in the magnetic field.

During the magnetization process, when the external magnetic field is gradually increased, the magnetic induction in the material also increases. However, when the external magnetic field is reduced, the magnetic induction does not immediately decrease to zero. Some amount of magnetic induction remains in the material even after the removal of the magnetic field.

Coercivity:
Coercivity is the measure of the resistance of a material to demagnetization. It is the amount of reverse magnetic field required to reduce the magnetic induction in the material to zero.

When the external magnetic field is reduced to zero, the magnetic induction in the material does not immediately become zero. The material retains some amount of magnetization. The coercivity of a material determines how easy or difficult it is to demagnetize the material.

Ferromagnetism:
Ferromagnetism is the property of certain materials to exhibit strong permanent magnetization. These materials have a high susceptibility to become magnetized in the presence of an external magnetic field.

Conclusion:
The magnetic induction left behind in a material after the magnetizing field has been removed is called retentivity. It is a measure of the residual magnetization in the material and is influenced by the material's coercivity and ferromagnetic properties.