An embryo is an animal or plant in its early stages of development.
A Human embryo was a being between the gestational age of two weeks and twelve weeks. Prior to this, it was called a zygote, and after this it was called a fetus. The length of time that another species spent in a similar stage varied depending on the length of the gestation.
Dr. Arik Soong stole a number of Augment embryos from Cold Station 12 in 2134. In the 22nd century, Cold Station 12 was a top-secret facility located on an asteroid and jointly operated and staffed by Starfleet and the Denobulans under the auspices of the Interspecies Medical Exchange. The facility was used to store hazardous viruses and pathogens for research purposes, as well as over 1,800 embryos of 20th century Augments from the Eugenics Wars. (ENT: "Borderland")
In 2154, the Klingons used embryos from genetically-engineered Human Augments to resequence a number of test subjects. Unfortunately, there were some unanticipated side effects, including the dissolving of their cranial ridges. Augment DNA was found to be more aggressive than the Klingons realized; nevertheless, the test subjects became stronger and more intelligent before their neural pathways unexpectedly started to degrade, and they died in agony (see Klingon Augments). (ENT: "Affliction")