Mental Illness Definition: What Is Mental Illness?

A mental illness definition can, perhaps surprisingly, be difficult to develop. The question "What is mental illness?" isn't always an easy one to answer. Being human can be an awful task sometimes. Events can get us down; relationships can take a toll. Sometimes people collapse under the weight of their thoughts and emotions, and when they do, they suffer immensely. But does such a breakdown mean mental illness or just extreme distress (Early Warning Signs of Mental Illness)? What is the line, for example, between prolonged sadness and depression or between anxiety and an anxiety disorder? Having a mental illness definition to work from is a helpful starting point in answering the question "What is mental illness?"

Mental Illness Definition in the DSM-5

Mental illnesses are diseases of the brain (McNally, 2011). Because it is difficult to specifically define mental illness, there's often misunderstanding about it as well as people who live with it. Stereotyped images, often negative and incorrect, cloud perception of what mental illness actually is. An important tool in defining mental illness, and thus understanding it, is the American Psychiatric Association's (2013) Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). This book is the accepted authority on psychiatric disorders, or mental illness.

Mental illness is a very broad term, encompassing a long list of mental illnesses and disorders. The DSM-5's official mental illness definition includes these key principles:

  • the distress is clinically significant (it meets all diagnostic criteria for the specific disorder);
  • there's a problem in the areas of thoughts, emotional regulation, and/or behavior that reflects dysfunction (impaired or abnormal functioning);
  • the person experiences significant distress or disability in social, occupational, educational, or other important activities;
  • a response to an extreme stressor, such as a loss, that is expected and culturally approved is not a mental disorder because rather than a dysfunction, it's an appropriate and temporary reaction to a stressor.

Additional criteria are required for a mental health challenge to be classified as a mental illness. Whiston (2005) emphasizes that mental illnesses are

  • enduring
  • inflexible
  • have a distinct frequency, as defined by the DSM-5
  • have a distinct duration, as defined by the DSM-5
  • severe in symptomology to cause a marked decrease in functioning and increase in distress

So what is mental illness? A mental illness definition describes the essence of what is going with someone: a mental illness is distress and dysfunction that goes deeper than a reaction to even extreme life stressors; a mental illness is a dysfunction that negatively affects someone's thoughts, emotions, and/or behaviors and interferes in their ability to live a full life in society.

A mental illness definition is important in understanding mental health problems. Discover the answer to the question - What is a mental illness?

Mental Illness Definition: Serious Mental Illness (SMI)

Like so many other things in life, mental illness is complex and multifaceted. There are many different types of mental illness, and they each have a unique impact on a person living with it. Some, but not all, mental illnesses are considered to be serious mental illnesses.

All mental illnesses must meet the above criteria to be defined as a mental illness. The criterion that separates a serious mental illness from others is severity, or degree of impact. An SMI creates significant impairment, whereas other mental illness has mild or moderate impairment.

Initially developed for the purposes of funding, grants, and insurance reimbursements, the concept of a serious mental illness included these specific disorders:

schizophrenia and other psychotic disorders
bipolar disorder
schizoaffective disorder
pervasive developmental disorders
depression in childhood and adolescence
panic disorder
posttraumatic stress disorder (PTSD)
• certain types of eating disorders

Many experts, though, don't limit themselves to a list when helping someone. They assess the impact of mental illness on the individual and consider it to be serious if it has a severe, negative impact on his/her life (How To Diagnose Mental Illness). Accordingly, the same mental illness, such as bipolar disorder, could be considered a serious mental illness in one person but not in another.

Mental Illness Definition Helps Understanding

The question "What is mental illness?" is an important one to ask in seeking to understand people, both ourselves and others. A good mental illness definition is one that highlights the fact that mental illness is an illness of the brain, is complex, impacts thoughts, emotions, and/or behaviors, and significantly impairs someone's functioning in life.

APA Reference
Peterson, T. (2019, October 23). Mental Illness Definition: What Is Mental Illness?, HealthyPlace. Retrieved on 2024, July 13 from

Last Updated: April 5, 2022

Medically reviewed by Harry Croft, MD

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