Source : Webster's Revised Unabridged Dictionary (1913)

Aril \Ar"il\, Arillus \A*ril"lus\, n. [From LL. arilli dry
   grapes, perh. fr. L. aridus dry: cf. F,. arille.] (Bot.)
   A exterior covering, forming a false coat or appendage to a
   seed, as the loose, transparent bag inclosing the seed or the
   white water lily. The mace of the nutmeg is also an aril.
   --Gray.

Source : WordNet®

aril
     n : fleshy and usually brightly colored cover of some seeds that
         develops from the ovule stalk and partially or entirely
         envelopes the seed