dementia 是一个医学名词，指的是痴呆症。
痴呆症是一种因脑部伤害或疾病所导致的渐进性认知功能退化，且此退化的幅度远高于正常老化的进展。记忆力和思维会退化，也会丧失自理生活的能力。
比如老人痴呆症 就是 senile dementia.
Dementia is a syndrome in which there is deterioration in memory, thinking, behaviour and the ability to perform everyday activities.