Kerma is a measure of energy transferred from radiation to matter and stands for kinetic energy released to matter. It is related to, but not the same as absorbed dose. Kerma is measured by the SI unit, gray.
Kerma measures the amount of energy that is transferred from photons to electrons per unit mass at a certain position. Absorbed dose, on the other hand, measures the energy deposited in a unit mass at a certain position.
At radiological energies, transfer and deposition of energy is virtually equal. However, at higher energies, a photon may interact with tissue in one position and create an electron that possesses enough energy to deposit energy at a location away from the interaction point.