A whistle is an instrument which produces sound from a stream of gas, most commonly air. It may be mouth-operated, or powered by air pressure, steam, or other means. Whistles vary in size from a small slide whistle or nose flute type to a large multi-piped church organ.

In association football, the referee is the person responsible for interpreting and enforcing the Laws of the Game during a match. The referee is the final decision-making authority on all facts connected with play, and is the match official with the authority to start and stop play and impose disciplinary action against players and coaches during a match.