What is a defrost cycle?

Frost free refrigerators run a heater against the evaporator at intervals to melt accumulated frost, which then drains away. When the heater, the bimetal thermostat or the defrost control fails, frost builds until it blocks airflow entirely, and the giveaway symptom is a customer who reports that unplugging the fridge for a day "fixes" it, because a manual thaw does the defrost cycle’s job. A fully frosted coil plus that story is a defrost diagnosis.