Some reptiles are viviparous—they give birth to live young—but most still lay eggs. Encyclopaedia Britannica defines viviparity as retaining and growing the fertilized egg inside the mother until the offspring can live independently, with the embryo typically nourished through a placenta or similar structure; it notes this pattern occurs in many reptiles as well as most mammals.
A related condition, ovoviviparity, appears in certain snakes: eggs are retained until they hatch inside the mother, and the embryo mainly uses yolk rather than continuous maternal nourishment. Live-bearing reproduction is especially common among lizards and snakes, while living turtles, crocodilians, and tuatara are egg-layers.