Abnormality (or dysfunctional behavior), in the vivid sense of something deviating from the normal or differing from the typical (such as an aberration), is a subjectively defined behavioral characteristic, assigned to those with rare or dysfunctional conditions. Behavior is considered abnormal when it is atypical, out of the ordinary, causes some kind of impairment, or consists of undesirable behavior. Who is normal or abnormal is a contentious issue in abnormal psychology.