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]

WordNet (r) 3.0 (2006):

carboxyl group
    n 1: the univalent radical -COOH; present in and characteristic
         of organic acids [syn: carboxyl, carboxyl group]