An abnormality is a characteristic that is outside of what is considered normal.
In psychology, the term abnormal refers to behavior that deviates from what is considered "normal."
For instance, chronic insomnia is a deviation from the normal human need and ability to sleep on a regular schedule. Likewise, ongoing anxiety or depression are deviations from the normal state of everyday, short-term mood deviations. Although all individuals differ in some regards and there is no absolute 'normal' typically in science abnormality refers to something that differs so much from the majority that it is clearly outside the average or 'normal' range.