Seasonal affective disorder (SAD) and seasonal depression are terms that until recently were used to describe a mood disorder in which people experience a normal mood for part of the year and a predictable period of depressed mood in the winter or summer.

Seasonal affective disorder is no longer thought of as a separate kind of mood disorder. When people have that kind of pattern to their depressive episodes it is now called recurrent major depressive disorder (MDD) with seasonal pattern.