The Collaborative International Dictionary of English v.0.48:
Endosteum \En*dos"te*um\, n. [NL., fr. Gr. ? + ? a bone.] (Anat.) The layer of vascular connective tissue lining the medullary cavities of bone. [1913 Webster]WordNet (r) 3.0 (2006):
endosteum n 1: vascular membrane that lines the inner surface of long bones