酸酐是具有两个酰基键合于同一氧原子上的有机化合物。 称为“酐”的原因因为它是由两个羧酸脱水而成。若两侧酰基由同种羧酸衍生而来则称为对称酸酐，分子式可表达为：(RC(O))₂O。对称酸酐命名取决于相应羧酸命名，即词缀“酸”改为“酸酐”。 因此(CH₃CO)₂O称为：“乙酸酐”（或***、醋酐）。混合酸酐（或不对称酸酐）以两侧酰基分别对应的羧酸命名，如：甲酸乙酸酐。低级酸酐遇水水解，但高级酸酐不溶于水。

An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the parent acid is a carboxylic acid, the formula of the anhydride being (RC(O))₂O. Symmetrical acid anhydrides of this type are named by replacing the word acid in the name of the parent carboxylic acid by the word anhydride. Thus, (CH₃CO)₂O is called acetic anhydride. Mixed (or unsymmetrical) acid anhydrides, such as acetic formic anhydride (see below), are known.