Psychology defines abnormal typically in the realms of abnormal behavior. There are three main criteria that are used in defining what is abnormal.
They are:
1. The behvaior poses a risk to either themself/others.
2.
The person exhibiting the behavior is
distressed by it.
3. It is statisically deviant.
Also, to a lesser extent, societal norms come into play in defining what is abnormal, but the risk of using this as a criterion increases the risk of stigmatizing.