Happiness

Happiness was an emotion.

The Seventh Doctor believed that happiness was "nothing" unless it existed "side by side" with sadness. (TV: The Happiness Patrol)

The Vardy were programmed to keep the colonists of the human colony planet happy at all times. When one of the colonists died of natural causes, her friends and family began to grieve. The Vardy, reading grief as the opposite of happiness, treated it like a virus and began killing the colonists as to them, eliminating sadness meant eliminating sad people. (TV: Smile)