It means giving birth to a well developed offspring that has been developed from an egg that has hatched within the mother before giving birth or just after being extruded from the mother. The hatched young receives no metabolic input from the mother, but is in a protected environment.
This is used by some fishes, including several sharks, and reptiles, and many invertebrates. In case of amphibians the young is born as a larva, and performs metamorphosis outside the body of the mother.
This article is a stub. You can help Wikipedia by expanding it.