Source : Webster's Revised Unabridged Dictionary (1913)

Endosperm \En"do*sperm\, n. [Endo- + Gr. ? seed.] (Bot.)
   The albumen of a seed; -- limited by recent writers to that
   formed within the embryo sac.

Source : WordNet®

endosperm
     n : nutritive tissue surrounding the embryo within seeds of
         flowering plants