Avalanche Rating
It is a MOSFET's ability to absorb short energy pulses exceeding its breakdown voltage without damage; it is given in millijoules. The voltage spike that appears when an inductive load is switched off can be safely dissipated by the device as long as it carries limited energy.
When a MOSFET's drain–source voltage exceeds the breakdown value it starts conducting and dissipates the energy of the spike in itself. This is designed behaviour: controlled breakdown limits the voltage and consumes the energy stored in the inductance. As long as the energy stays below the device's declared avalanche limit, no permanent damage occurs.
What sets the limit is heat. Because the energy is released in a very short time over a small die area, the temperature rises quickly; when the limit is exceeded, local melting and a permanent short circuit occur. For this reason avalanche is not a continuous protection method but a safety margin against single and infrequent events.
In practice, rather than relying on that margin, it is preferable to limit the spike in the first place: freewheeling diodes, snubber circuits and suppression devices at the supply input do this job. The part that falls to you in the field is the wiring; keeping the motor and supply cables short directly reduces the loop inductance and therefore the energy released at every switching event.
The context in which this term is used: DC Motor Driver Selection Guide