The Collaborative International Dictionary of English v.0.48:

Acetyl \Ac"e*tyl\, n. [L. acetum vinegar + Gr. ? substance. See
   -yl.] (Chem.)
   A complex, hypothetical radical, composed of two parts of
   carbon to three of hydrogen and one of oxygen. Its hydroxide
   is acetic acid.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acetyl
    n 1: the organic group of acetic acid (CH3CO-) [syn: acetyl,
         acetyl group, acetyl radical, ethanoyl group,
         ethanoyl radical]