A Mental Disorder is a psychological behavioral syndrome occurring in a person that results in clinically significant impairment or distress, not an expectable response to a particular event and not a manifestation of cultural norms
Every phrase of this definition is significant:
psychological behavioral: A diagnosis of a mental disorder occurs where the psychological or behavioral symptoms are the most prominent symptoms
syndrome: A pattern or cluster of symptoms that tend to occur together occurring in a person: An individual, not societal problem.
clinically significant impairment or distress: There is a difference between unconventional behavior and a mental disorder. People shouldn't be diagnosed with a mental disorder just because they're 'different'. It is acknowledged that in some cases there is a blurred line between normality and abnormality, and that diagnosis has a subjective component. Care should be taken not to over-pathologize behavior.
not an expectable response to a particular event and not a manifestation of cultural norms