To disclose information to the public or to appropriate authorities concerning the illegal or socially harmful actions of a person or group, especially a corporation or government agency
In some jurisdictions, it is illegal to fire a person for blowing the whistle on an employer.
cause an action or situation to stop by revealing it to opposing parties; blow a whistle to indicate the start or finish of a game or a time-out in an activity
(deyim) Bring (an illicit activity) to an end by informing on the person responsible; call public or official attention to something (as a wrongdoing) kept secret ― usually used with on