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]