A structure of cells (indeed a small sprout of cells) formed in the early embryonic development of mammals. It possesses an inner cell mass also known as the embryoblast (which subsequently forms the embryo) and an outer layer of trophoblast cells called the trophectoderm—this layer encases the inner cell mas and a cavity/sac of fluid. In the stages of embryological development, the blastocyst develops after the blastula and later implants into the uterine lining.

The blastula is subtly different from a blastocyst in that a blastocyst is considered to have two distinct cell types: an inner cell mass (which will later become the embryo) and the trophoblast cells (which will later become the placenta).