r-co-

from The Collaborative International Dictionary of English v.0.48

Acyl \Ac"yl\, n. [Acid + -yl.] (Org. Chem.)
   An acid radical, as acetyl, malonyl, or benzoyl. An acyl
   radical can be depicted as {R-CO-}, where {-CO-} is the
   carbonyl group, and R is the group that characterizes the
   acyl moiety.
   [Webster 1913 Suppl. +PJC]