Source : Webster's Revised Unabridged Dictionary (1913)

Carboxyl \Car*box"yl\, 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}.

Source : WordNet®

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

carboxyl
     adj : relating to or containing the carboxyl group or carboxyl
           radical [syn: {carboxylic}]