Abnormality (or dysfunctional behavior) is a behavioral characteristic assigned to those with conditions that are regarded as rare or dysfunctional. Behavior is considered to be abnormal when it is atypical or out of the ordinary, consists of undesirable behavior, and results in impairment in the individual's functioning. Abnormality in behavior, is that in which is considered deviant from specific societal, cultural and ethical expectations. These expectations are broadly dependent on age, gender, traditional and societal categorizations. The definition of abnormal behavior is an often debated issue in abnormal psychology, because of these subjective variables.

Abnormal behavior should not be confused with unusual behavior. Behavior that is out of the ordinary is not necessarily indicative of a mental or psychological disorder. Abnormal behavior, on the other hand, while not a mental disorder in itself, is often an indicator of a possible mental and/or psychological disorder. A psychological disorder is defined as an "ongoing dysfunctional pattern of thought, emotion, and behavior that causes significant distress, and is considered deviant in that person's culture or society". Abnormal behavior, as it relates to psychological disorders, would be "ongoing" and a cause of "significant distress". A mental disorder describes a patient who has a medical condition whereby the medical practitioner makes a judgment that the patient is exhibiting abnormal behavior based on the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria. Thus, simply because a behavior is unusual it does not make it abnormal; it is only considered abnormal if it meets these criteria. The DSM-5 is used by both researchers and clinicians in diagnosing a potential mental disorder. The criteria needed to be met in the DSM-5 vary for each mental disorder.

Unlike physical abnormalities in one's health where symptoms are objective, psychology health professionals cannot use objective symptoms when evaluating someone for abnormalities in behavior.