An acid anhydride is a carboxylic acid derivative formed by joining two acyl groups (R–CO– units) through a single oxygen atom, written R–CO–O–CO–R′. The name 'anhydride' means 'without water', reflecting that it is formally two acid molecules condensed with loss of water. Anhydrides are reactive acylating agents, intermediate in reactivity between the very reactive acyl chloride and the less reactive ester, and are named with '-oic anhydride'.

酸酐是一类羧酸衍生物,由两个酰基(R–CO– 单元)通过一个氧原子连接而成,写作 R–CO–O–CO–R′。“anhydride(酐)”意为“不含水”,反映出它在形式上是两分子酸缩去一分子水而成。酸酐是活泼的酰化试剂,其反应性介于很活泼的酰氯与较不活泼的酯之间,以“-oic anhydride(酸酐)”命名。