Source : Webster's Revised Unabridged Dictionary (1913)

Hydroxyl \Hy*drox"yl\, n. [Hydro-, 2 + oxygen + -yl.] (Chem.)
   A compound radical, or unsaturated group, {HO}, consisting of
   one atom of hydrogen and one of oxygen. It is a
   characteristic part of the hydrates, the alcohols, the oxygen
   acids, etc.

Source : WordNet®

hydroxyl
     n : the monovalent group -OH in such compounds as bases and some
         acids and alcohols [syn: {hydroxyl group}, {hydroxyl
         radical}]