Here is a definition I found on the internet:
A mental disorder, also called a mental illness or psychiatric disorder, is mental or behavioral pattern that causes either suffering or a poor ability to function in ordinary life. Many disorders are described.[1] Conditions that are excluded include social norms. Signs and symptoms depend on the specific disorder.[1]
The causes of mental disorders are often unclear. Theories may incorporate findings from a range of fields. Mental disorders are usually defined by a combination of how a person feels,