Behavioral health is the scientific study of the emotions, behaviors and biology relating to a person's mental well-being, their ability to function in every day life and their concept of self. It is also  the level of psychological well-being or an absence of mental illness. It is the state of someone who is "functioning at a satisfactory level of emotional and behavioral adjustment". Behavioral health describes the connection between behaviors and the health and well-being of the body, mind and spirit. This would include how behaviors like eating habits, drinking or exercising impact physical or mental health. Behavioral health refers to how our daily cognitive habits affect our overall well-being, emotions, biology, and behavior. It's often used interchangeably with mental health, but is actually a far more expansive term that incorporates not just our mental wellness, but the way our thoughts play out in real life. 

So over all, Behavioral health is defined as the connection between behaviors and the health and well-being of the body, mind and spirit. This includes a wide variety of healthcare services, including:

Mental healthcare Psychiatric care Marriage and family counseling Substance abuse prevention, intervention, treatment, and recovery Management of chronic diseases

So while mental health related illnesses and treatments are a branch of behavioral health.

The link between behavioral health and mental health varies from case to case, patient to patient. For example, people who suffer from diabetes or chronic cardiac conditions often develop depression as well. On the other hand, those who suffer from depression may have no outward physical signs of illness—making their treatment mental health-based, instead of behavioral. Regardless, many mental health conditions develop from biological factors—some manageable, like brain chemistry, and others out of our control, like genetic makeup. Cognitive behavioral therapy and medicine can often help alleviate immediate symptoms, but so too can something as simple as learning how to think in a healthier way regarding one’s mental illness.