Sadness is an emotion. It is the opposite of happiness. People feel sad when something bad has happened, for example, if their mother or father has died, or if they are parted from friends. The word "miserable" has a similar meaning.
In big letters, SAD, or S.A.D. is short for "Seasonal Affective Disorder". SAD is a sickness that some people have in the winter. They feel depressed as the nights become longer and there is less daylight.
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It is normally thought of as the opposite of happiness, but you can feel both happy and sad at the same time.