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]

WordNet (r) 3.0 (2006):

acyl
    n 1: any group or radical of the form RCO- where R is an organic
         group; "an example of the acyl group is the acetyl group"
         [syn: acyl, acyl group]