The Collaborative International Dictionary of English v.0.48:

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]

The Collaborative International Dictionary of English v.0.48:

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]

The Collaborative International Dictionary of English v.0.48:

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]