Sad is characterized as a negative emotion  or feeling
1.   affected by unhappiness or grief; sorrowful or mournful:
“to feel sad because a close friend has moved away.”

2.   expressive of or characterized by sorrow:
“sad looks; a sad song.”

3.   causing sorrow:
“a sad disappointment; sad news.”

Synonyms:   unhappy, despondent, disconsolate, discouraged, gloomy, downcast, downhearted, depressed, dejected, melancholy.

Antonyms:   happy.

Most psychotherapists would agree that depression is anger and sadness that has been repressed. Sadness is an emotion, and depression is an illness. And whereas many measures of depression and sadness have been defined, a coherent description of happiness remains elusive.