Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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