Hysteresis loss in electrical machines is the energy lost as heat due to the lag between the magnetic field and the magnetization of the material. When the magnetic material (like the iron in a transformer or motor) is exposed to a changing magnetic field, the magnetic domains inside it don’t align and realign instantly. This delay causes some energy to be dissipated as heat every time the magnetic field changes direction.

You can think of it like trying to push a heavy door back and forth. It doesn’t snap back instantly—you have to exert extra effort each time, and that effort gets wasted as friction. In the case of electrical machines, the “friction” is the resistance of the material to changing its magnetic state. This loss depends on the frequency of the magnetic changes and the properties of the material.