Abnormal behavior can refer to or best described as a disturbance of an individual's behavioral psychological , or physical functioning that is not culturally expected and that leads to psychological distress, behavioral disability or impaired overall functioning.
The patterns of emotion, thought, and action deemed pathological for one or more of the following reasons: infrequent occurrence, violation of norms, personal distress, disability or dysfunction, and unexpectedness.
Other definition:
Abnormal behavior refers to a person's actions that are unexpected and often evaluated negatively because they differ from typical or usual behavior.