What is the difference between sadness and depression?

Sadness and depression are often confused, but they are fundamentally different:

Sadness: — A normal, healthy emotion — Has an identifiable cause (a loss, a disappointment, a difficult change) — Is proportional to that cause — Fades over time, or once the situation changes — Does not completely prevent functioning — The person can still feel moments of pleasure

Depression: — A clinical condition, not just an emotion — Often has no clear external cause, or persists far beyond what the cause would justify — Lasts at least 2 weeks (the minimum criterion), but often much longer — Significantly impairs functioning (work, relationships, self-care) — Anhedonia (the inability to feel pleasure) is central; nothing feels enjoyable — Includes physical symptoms (sleep, appetite, energy) and cognitive symptoms (concentration, memory) — Does not simply improve because good things happen

Grief is a special case: intense sadness after a significant loss is normal and is not depression, even when it is intense. But grief can evolve into depression, especially when it persists for a long time or includes feelings of worthlessness, thoughts of death, or a prolonged inability to function.