What is energy stored in an inductor?

Hint: Energy in physics is defined as the ability to do work. Inductor is defined as a two terminal electrical component that stores energy in a magnetic field when current flows through it. In terms of formula energy stored in capacitor refers to the product of half of inductance and the square of current through it.

Complete step-by-step solution:
One explanation is presented as why a potential difference is induced on a change of current in an inductor.
When there is a change in the flow of current through an inductor then there is a change in the strength of the magnetic field. For example, if the current is increased, the magnetic field also increases and vice-versa. However, this does not come without any loss. The magnetic field contains potential energy and increasing the field strength requires more energy that has to be stored in the field. This energy comes from the electric current through the inductor. The increase in the magnetic potential energy of the field is provided by a corresponding drop in the electric potential energy of the charges flowing through the loops or winds in any body. This appears as a voltage drop across the windings as long as the current increases. Then there appears a state where the current is no longer increased and becomes constant, the energy in the magnetic field is also constant. At that phase no additional energy has to be supplied, so the voltage drop across the windings vanishes.
Similarly, if the current through the inductor decreases, the magnetic field strength decreases, and the energy in the magnetic field decreases. This energy is returned to the circuit in the form of an increase in the electrical potential energy of the moving charges, causing a voltage rise across the windings.
This is actually the energy that is stored in the inductor.

Note: The question overviews what actually energy stored in an inductor implies rather than the general formula. An inductor is characterized by its inductance which is the ratio of the voltage to the rate of change in current.