A hitman is someone who gets paid to kill a specific person. A member of a criminal organization, like the mafia, might be arrested after hiring a hitman to kill an enemy.
If a person's job involves professional murder for pay, that person is a hitman. It's an informal term for an uncommon occupation, one you're most likely to encounter in crime novels and movies. While these hired killers have existed in the U.S. at least since the organized crime heyday of the 1920s, the term itself only dates to the 1970s, from the underworld crime sense of hit meaning "to kill by plan."