The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hydroxyl
    n 1: the monovalent group -OH in such compounds as bases and
         some acids and alcohols [syn: hydroxyl, hydroxyl group,
         hydroxyl radical]