The Collaborative International Dictionary of English v.0.48:

Allyl \Al"lyl\, n. [L. allium garlic + -yl.] (Chem.)
   An organic radical, C3H5, existing especially in oils of
   garlic and mustard.
   [1913 Webster]

WordNet (r) 3.0 (2006):

allyl
    n 1: the univalent unsaturated organic radical C3H5; derived
         from propylene [syn: allyl, allyl group, allyl
         radical]