A deviant is someone whose behavior falls far outside of society's norms; as an adjective, deviant can describe the behavior itself. For example, a fifty-year-old punk rocker has a deviant appearance, compared to his peers.
That aging punk deviates, or departs from the norm, of people his age. Both deviant and deviate come from the Latin word deviare, meaning "to turn out of the way." Deviants experience this turning away from society, either by choice, or in the case of someone who has harmed another, perhaps by sexual abuse, by being cast out by people who want nothing to do with them.