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]