癡呆癥是壹種因腦部傷害或疾病所導致的漸進性認知功能退化，且此退化的幅度遠高於正常老化的進展。記憶力和思維會退化，也會喪失自理生活的能力。
比如老人癡呆癥 就是 senile dementia.
Dementia is a syndrome in which there is deterioration in memory, thinking, behaviour and the ability to perform everyday activities.