1 a : a deranged state of the mind usually occurring as a specific disorder (as schizophrenia) and usually excluding such states as mental retardation, psychoneurosis, and various character disorders b : a mental disorder
2 : such unsoundness of mind or lack of understanding as prevents one from having the mental capacity required by law to enter into a particular relationship, status, or transaction or as removes one from criminal or civil responsibility
3 a : extreme folly or unreasonableness b : something utterly foolish or unreasonable