If you do something intentionally, you do it on purpose. You chose to do it. Like “intentional,” this word often appears when the act hurts someone: someone intentionally left you out, or intentionally broke a rule. In court, “intentionally” is strong — it means the person meant the harm. Though actually, people also use it the other way for small mistakes: “I didn’t do it intentionally” just means “I didn’t mean to.” Put it near the verb: “She intentionally ignored the message.” In everyday talk, “on purpose” is the simple twin.