Medical Definition of Affective disorder, seasonal (SAD)
Affective disorder, seasonal (SAD): Depression that tends to occur (and recur) as the days grow shorter in the fall and winter. It is believed that affected persons react adversely to the decreasing amounts of light and the colder temperatures as the fall and winter progress.
Seasonal affective disorder has not been recognized very long as a medical condition. The term first appeared in print in 1985. Seasonal affective disorder is also sometimes called winter depression or the hibernation reaction.