There is no technical definition for mental illness or mental disorder. Psychologists avoid using the terms because it suggests a difference between mental disorders and physical disorders, when there isn't really much difference. The mind and body are too closely linked to have separate disorders.
Any of various conditions which cause problems with a person's normal thinking, feeling, or behavior, and caused by social, psychological, biochemical, genetic, or other factors, such as infection or head trauma. Also called emotional illness, mental disease, mental disorder.