Duty to warn refers to mental health professional’s responsibility to break confidentiality and notify the potential victim whom a client (patient) has specifically threatened.
Other /More definition:
Duty to warn refers to the clinician's/therapist's responsibility to notify a potential victim of a client's harmful intent toward that individual. It is the legal assumption that a clinician must make all efforts to prevent a client from danger to another or to self. Duty to warn, moreover, is the mental health professional’s responsibility to break confidentiality and notify the potential victim whom a client has specifically threatened.