Anhydride refers to a compound that readily reacts with water to form an acid or base. If the product of the reaction with water gives hydrogen ions, characteristic of acids, the original substance is an acid anhydride. Examples include acid anhydrides like acetic anhydride (CH3CO)2O and phthalic anhydride. Anhydrides are formed by removing a water molecule from two molecules of a parent acid. 

Oxides of non-metals are commonly referred to as acid anhydrides because they react with water to form acidic solutions or acids. Hence the solutions of their oxides in water has a pH less than 7.