An anhydride is a chemical compound obtained by the removal of water from another compound.
Acidic anhydride is an oxide that reacts with water to form an acid.
Thus acidic anhydrides are acids with water removed. A basic anhydride is an oxide of a metal that reacts with water to give an aqueous hydroxide, or that reacts with protic acids to give aqueous metal salts. These can sometimes be formed by thorough dehydration of hydrous oxides or hydroxides.