Punishment is the penalty you have to pay when you're caught doing something bad. A teenager's punishment for missing her curfew might be helping her dad clean the garage.
When someone is officially penalized for a mistake or a crime, that's punishment. Stealing a car could result in a punishment involving jail, while pinching your little sister might mean a punishment as mild as sitting in a chair and thinking about ways to be nicer next time. The Latin root of punishment and its related verb, punish, is punire, "punish, correct, take vengeance for, or cause pain for some offense."