There are several ways to define forensic psychiatry. One useful way has been developed by a European study group in forensic psychiatry, called the Ghent group. they called it “a specialty of medicine, based on a detailed knowledge of relevant legal issues, criminal and civil justice systems. Its purpose is the care and treatment of mentally disordered offenders and others requiring similar services, including risk assessment and management, and the prevention of future victimisation.”
This is a bit long winded but it tells you that it is a branch of psychiatry which has a central concern with criminal behaviours in those with significant psychological and psychiatric dysfunction (Mullen and Ogloff).