A shark is a skilled player who profits from weaker opponents, mainly in games played against other people, such as poker. The term is the counterpart of fish, the inexperienced players a shark looks for, and a pool shark does the same at billiards. In its sharper sense, it describes a hustler who hides their ability, acting clumsy or unsure of the rules to lure others into bigger bets before revealing their real strength.
For operators of peer-to-peer games, sharks are a question of balance as much as a slang term. Poker networks protect recreational players through measures such as anonymous tables, limits on third-party tools, seating rules and collusion detection, because a healthy game needs newcomers who enjoy themselves and come back.