The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

hydroxy acid
    n 1: any acid that has hydroxyl groups in addition to the
         hydroxyl group in the acid itself