Anhydride

A chemical compound derived from an acid by elimination of a molecule of water. Thus, sulfur trioxide (SO3) is the anhydride of sulfuric acid (H2SO4), carbon dioxide (CO2) is the anhydride of carbonic acid (H2CO3), and phthalic acid [C6H4(CO2H)2] minus water gives phthalic anhydride [C6H4(CO2)O]. Not to be confused with anhydrous