insanity

n.

1. Mental derangement

   mental illness, mental disorder, derangement, psychopathy, madness, dementia, amentia, lunacy, psychosis, alienation, neurosis, psychoneurosis, phobia, mania, craziness, aberration; see also complex 1, disease.

   Mental illnesses and disorders include: schizophrenia, dementia praecox, catatonia, paranoia, hysteria, catalepsy, manic-depressive disorder, bipolar disorder, depression, melancholia, cyclothymia, delusion, hallucination, delirium, amnesia, split personality, multiple personality, autism, mental retardation, senile dementia, obsession, fixation, compulsion, obsessive-compulsive disorder, mania, phobia; nervous breakdown*.

2. Disorders usually accompanied by impaired mental functioning include: Down syndrome, Mongolism*, cretinism, microcephaly, hydrocephaly, Alzheimer's disease.

   Antonyms sanity*, reason*, normality. *

3. Utter folly

   senselessness, foolishness, foolhardiness; see stupidity 2.

insanity, current in popular and legal language but not used technically in medicine, implies mental derangement in one who formerly had mental health; lunacy specifically suggests periodic spells of insanity, but is now most commonly used in its extended sense of extreme folly; dementia is the general term for an acquired mental disorder, now generally one of organic origin, as distinguished from amentia (congenital mental deficiency); psychosis is the psychiatric term for any of various specialized mental disorders, functional or organic, in which the personality is seriously disorganized