The American Heritage® Dictionary of the English Language, 4th Edition
n. The outermost layer of cells of the blastocyst that attaches the fertilized ovum to the uterine wall and serves as a nutritive pathway for the embryo. Also called trophoderm.
n. The membrane of cells that forms the wall of a blastocyst during early pregnancy, providing nutrients to the embryo and later developing into part of the placenta.
The Century Dictionary and Cyclopedia
n. An external epiblastic layer that does not enter into the formation of the embryo, but does take an active part in nutritional processes intended for it; the blastocystic ectoderm.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. the membrane that forms the wall of the blastocyst in early development; aids implantation in the uterine wall
Word Usage
"A typical day-5 human embryo consists of 200-250 cells, most of which comprise the trophoblast, which is the outermost layer of the blastocyst."