An anhydride is a compound formed by eliminating water from another compound.

In organic anhydride, it has two acyl groups bonded to the same oxygen atom.

Example of an inorganic anhydride SO₃ which is derived from sulfuric acid and also a compound CaO derived from calcium hydroxide, an example of an organic anhydride is acetic anhydride (CH₃CO)₂O derived from acetic acid.