C2O2

from The Collaborative International Dictionary of English v.0.48

Oxalyl \Ox"a*lyl\, n. [Oxalic + -yl.] (Chem.)
   (a) A hydrocarbon radical ({C2O2}) regarded as a residue of
       oxalic acid and occurring in derivatives of it.
   (b) An old name for {carbonyl}.
   (c) An old name for {carboxyl}.
       [1913 Webster]