- 1 (archaic): 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.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
- Having to do with an embryo, which is an early stage in the development of a plant or animal.
National Cancer Institute
Related discussion in the Handbook
See also Understanding Medical Terminology.