Anhydride, a chemical compound that can be formed by removing water from another compound. In organic chemistry, the most important anhydride is acetic anhydride, a compound formed by removing water from acetic acid. Acetic anhydride is used mainly to make cellulose acetate, from which a synthetic textile fiber (acetate), photographic film, and magnetic-recording tape are produced. Organic anhydrides react with water to form organic acids.

In inorganic chemistry, the most common type of anhydride is an oxide (a compound consisting of oxygen and one other element). Some oxides of nonmetals (such as phosphorus and sulfur) react with water to form inorganic acids. Such oxides (and all organic anhydrides) are known as acid anhydrides. Some metallic oxides (such as those of sodium and potassium) form bases when they unite with water, and are called basic anhydrides.