-COOH

Carboxyl \Car*box"yl\ (k[aum]r*b[o^]ks"[i^]l), n. [Carbon +
   oxygen + -yl.] (Chem.)
   The complex radical, {CO.OH}, regarded as the essential and
   characteristic constituent which all oxygen acids of carbon
   (as formic, acetic, benzoic acids, etc.) have in common; --
   called also {oxatyl}.
   [1913 Webster]
    

carboxyl group \car*box"yl group`\ (k[aum]r*b[o^]ks"[i^]l
   gr[=oo]p), n. (Chem.)
   a monovalent organic radical (written {-COOH}, {-CO2H} or
   {-CO.OH}) whose presence confers acidic properties on the
   compound containing it; -- called also {carboxylic group}.
   Monocarboxylic acids (such as acetic acid or butyric acid)
   have one carboxyl group, dicarboxylic acids (such as oxalic
   acid or malonic acid) have two, tricarboxylic acids (such as
   citric acid) have three. It is weakly acidic, the pKa of
   acetic acid being 4.75.
   [PJC]
    

carboxylic \car`box*yl"ic\ (k[aum]r`b[o^]ks*[i^]l"[i^]k), adj.
   (Chem.)
   of or pertaining to the {carboxyl group}; having a carboxyl
   group (written {-COOH}, {-CO2H} or {-CO.OH}); as, carboxylic
   acids are weakly acidic.

   Syn: carboxyl.
        [WordNet 1.5]