Illustrated Glossary of Organic Chemistry

Anhydride (acid anhydride): A functional group characterized by two acyl groups joined by an oxygen atom. When the R groups in the general structure shown below are not the same (i.e., when the anhydride is derived from two different carboxylic acids) it is called a mixed anhydride.

Anhydride general molecular structure.
Acetic anhydride.   
Acetic benzoic anhydride, a mixed anhydride.