Punishable means deserving or capable of being punished. When something is punishable, there's a rule or law saying that people who do it can face consequences.

Schools have rules about behavior, and breaking them is punishable: talking during a test might be punishable by losing recess time, while repeatedly ignoring homework might be punishable by detention. In society, crimes are punishable by law. Stealing is punishable, speeding is punishable, and more serious crimes carry more serious punishments.

The word often appears with “by” to specify the consequence: “Cheating on the exam is punishable by failing the course.” Sometimes you'll see it describe the maximum penalty: “The offense is punishable by up to three months in jail.”

Notice that punishable doesn't mean someone will be punished, just that they can be. A teacher might decide that a minor infraction is punishable but choose to give a warning instead. The word describes what's allowed or appropriate under the rules, not what actually happens in every case.