Insanity, lunacy, psychosis, mania, dementia are the leading general terms denoting serious mental disorder.
Insanity as a technical term belongs to law rather than to medicine. It is used to cover a wide variety of mental disorders, all of which have in common one characteristic—an unfitting of the afflicted individual to manage his own affairs or perform his social duties.
Mental deficiency and delirious conditions are usually excluded, the former as inborn and not acquired, the latter as temporary and not long-lasting. Since in law a person’s sanity or insanity becomes an issue when he is charged with a crime or when his legal capacity to make a will or contract or to transfer property is questioned, proof of insanity is tantamount to proof of his inability to act rationally and to understand the nature of his act and its natural consequences in affecting his rights, obligations, and liabilities. In general use insanity is commonly distinguished from mental deficiency and from neuroses and is applied to disorders involving unsoundness or derangement of mind.
Lunacy in general use often applies to insanity manifested in spells of madness and fury or interrupted by intervals of lucidity.
Lunacy sometimes is used interchangeably with insanity in law <a lunacy commission> <filed a lunacy petition against the attorney general so that a court could pass on his mental condition —Time >
Psychosis is the psychiatric term for a profound disorganization of mind, personality, or behavior resulting from an individual’s inability to cope with his environment. Though in content often coextensive with insanity or lunacy it carries none of the special implications of these two terms.
Mania (for fuller treatment see MANIA 2 ) denotes a phase marked by sustained and exaggerated elation, excessive activity (as in emotional expression or physical action), or delusions of greatness that characterizes certain psychoses.
Dementia implies a marked decline from a former level of intellectual capacity often accompanied by emotional apathy and is applicable to most psychoses that involve organic deterioration, not only those manifesting themselves in spells of excitement but those manifesting themselves in apathy, depression, flightiness, or personality disintegration.