Clinical Psychology - The branch of apllied psychology that deals with understanding and treating mental disorder
Statistical Definition Of Abnormality - The most literal way of defining something as abnormal is how often it occurs. Anyhting that occurs relatively rarely can be seen as abnormal.
Social Norms Definiton of Abnormality - There are commonly accepted standards of behaviour, some are written laws (no crime etc), and there are some unwritten customs, such as Queueing in the UK.
If someone was to deviate from these norms, it would be an indicator of abnormality. E.g. someone with OCD may wash their hands 50 times per day.
Schizophrenia - A psychcotic disorder which can leave the person out of touch with reality.
Reliability - The consistency with which a measure of a psychological variable like mental disorder identifies the same thing