1) Membrane 2) Tissue layer 
In anatomy the endosteum (plural endostea) is a thin layer of connective tissue that lines the surface of the bony tissue that forms the medullary cavity of long bones. This endosteal surface is usually resorbed during long periods of malnutrition, resulting in less cortical thickness. The outer surface of a bone is lined by a thin layer of connec...
• (n.) The layer of vascular connective tissue lining the medullary cavities of bone.
a layer of bone forming cells, osteoblasts which covers the entire surface of the internal aspect of cortical and spongy bone, separating it from the surrounding connective tissue. see also bone membrane.
(en-dos´te-әm) the tissue lining the medullary cavity of a bone.
En·dos'te·um noun [ New Latin , from Greek ... + ... a bone.] (Anat.) The layer of vascular connective tissue lining the medullary cavities of bone. 
Type: Term Pronunciation: en-dos′tē-ŭm Definitions: 1. A layer of cells lining the inner surface of bone in the central medullary cavity. Synonyms: medullary membrane, perimyelis
The endosteum is the layer of bone which lines the marrow cavity within the long bones which houses the bone marrow. It is distinguished from the periosteum, which is the external membrane which covers the surface of the compact bone tissue.
[n] - vascular membrane that lines the inner surface of long bones
noun vascular membrane that lines the inner surface of long bones
Internal periosteum; the membrane lining the medullary cavity
the membrane lining the medullary cavity of a bone.
Tissue surrounding the medullary cavity of bone.