Latin for 'away from mind' — from 'de-' + 'mens' (mind), the same PIE root behind 'mental,' 'memory,' and 'mania.'
A chronic or persistent disorder of the mental processes caused by brain disease or injury, marked by memory loss, personality changes, and impaired reasoning.
From Latin 'dēmentia' (madness, insanity, folly, loss of mind), from 'dēmēns' (out of one's mind, mad, senseless), a compound of 'dē-' (away from, down from, completely, a marker of deprivation or removal) + 'mēns' (mind, intellect, reason, understanding, purpose), from PIE *men- (to think, to have in mind). PIE *men- is one of the most prolific roots in the Indo-European family: it gives Latin 'mēns' (mind), 'mentis,' 'mentio' (mention — what comes to mind), 'monitor' (one who reminds), 'monument' (memory marker), 'admonish,' 'premonition,' and 'comment.' In Greek, the same root gives 'menos' (spirit, force of mind), 'mania' (madness), 'mantis' (prophet
The PIE root *men- (to think) produced an astonishing range of English words through different branches: 'mind' (Germanic), 'mental' and 'dementia' (Latin 'mēns'), 'memory' and 'remember' (Latin 'memor'), 'mania' (Greek 'manía'), 'mantra' (Sanskrit), 'automatic' (Greek 'automatos,' self-thinking), and even 'monster' (Latin 'monēre,' to warn — a monster was originally a divine warning sign, something that made you think).