1) A vertebrate at any stage of development prior to birth or hatching.

2) An animal in the early stages of growth and differentiation that are characterized by cleavage, the laying down of fundamental tissues, and the formation of primitive organs and organ systems; especially: the developing human individual from the time of implantation to the end of the eighth week after conception.

3) Having to do with an embryo, which is an early stage in the development of a plant or animal.