An easy way to understand the term is through the opposite of normal - which is "pathological" or abnormal.

There are four different criteria for defining abnormality:

• Statistic approach. It is based on the idea that normality is what is most common. But it loses effectiveness when there are many variables.
• Biological approach. In this approach, behaviors or processes follow biological normality. But the problem with these criteria is that biological laws can be wrong or incomplete.
• Social approach. It's based on the idea that normality is what society accepts as normal.
• Subjective approach. If a person considers their behavior as abnormal, it is enough to consider them as such.

Although these criteria are used to diagnose and treat disorders in clinical psychology, we can use them to better understand our personal concept of normal and abnormal.