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: Webster's Revised Unabridged Dictionary (1913)