To understand what abnormal behavior is, we must first understand what it means to be normal. The word normal simply means average. So, normal people are those who behave in an average way. Statistically speaking, normal behavior is the behavior which we would expect to see in most people. Therefore, abnormal behavior is that behavior which does not occur in the majority of people.
In the practice of psychology, the term abnormal behavior has taken on a further meaning of referring to mental illness.