Seasonal affective disorder (SAD), also called winter depression, winter blues, summer depression, and seasonal depression, is a mood disorder subset in. Define sad. sad synonyms, sad pronunciation, sad translation, English dictionary definition of sad. abbr. seasonal affective disorder adj. sad·der , sad·dest 1. Sad definition, affected by unhappiness or grief; sorrowful or mournful: to feel sad because a close friend has moved away. See more. SAD is an Indie dedicated audiovisual curation blog. Based in Amsterdam. — Submissions: [email protected] All rights reserved to the artists featured. Define sad: affected with or expressive of grief or unhappiness : downcast — sad in a sentence Seasonal affective disorder (SAD) is a type of depression that’s related to changes in seasons and begins and ends at about the same times every year.