Public health is 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" From the perspectives of positive psychology or of holism, mental health may include an individual's ability to enjoy life and to create a balance between life activities and efforts to achieve psychological resilience. According to the World Health Organization (WHO), mental health includes "subjective well-being, perceived self-efficacy, autonomy, competence, inter-generational dependence, and self-actualization of one's intellectual and emotional potential, among others". The WHO further states that the well-being of an individual is encompassed in the realization of their abilities, coping with normal stresses of life, productive work, and contribution to their community. Cultural differences, subjective assessments, and competing professional theories all affect how one defines "mental health

According to the U.K. Surgeon Journal (1999), mental health is the successful performance of the mental function resulting in productive activities, fulfilling relationships with other people, and providing the ability to adapt to change and cope with adversity. The term mental illness refers collectively to all diagnosable mental disorders—health conditions characterized by alterations in thinking, mood, or behavior associated with distress or impaired functioning. Mental health and mental illness are two continuous concepts. People with optimal mental health can also have a mental illness, and people who have no mental illness can also have poor mental health.

Mental health problems may arise due to stress, loneliness, depression, anxiety, relationship problems, death of a loved one, suicidal thoughts, grief, addiction, ADHD, self-harm, various mood disorders, or other mental illnesses of varying degrees, as well as learning disabilities. Therapists, psychiatrists, psychologists, social workers, nurse practitioners, or family physicians can help manage mental illness with treatments such as therapy, counseling, or medication