Hydroxy- \Hy*drox"y-\ (Chem.) A combining form, also used adjectively, indicating hydroxyl as an ingredient. [1913 Webster] {Hydroxy acid} (Chem.), an organic acid, having (besides the hydroxyl group of the carboxyl radical) an alcoholic hydroxyl group, and thus having the qualities of an alcohol in addition to its acid properties; as, lactic and tartaric acids are hydroxy acids. [1913 Webster]