What is commutation diode?

Commutation is a scenario when the operating scenario switches into a different polarity (phase). For example, take the case of a diode which is used in parallel to the coil (a snubber diode or fly back diode). The diode is OFF when the load is connected to the cool and supply at the other end. When the load is diconnected, the energy in the coil has to be dissipated, this is where the diode conducts and acts as a load for the coil during that instant. Another scenario of commutation diode is the bridge rectifier where diodes are ON during one phase of the input and the same diodes are OFF during the other phase of the input.