The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
endosperm n 1: nutritive tissue surrounding the embryo within seeds of flowering plants