In zoology, an egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg (Latin, ovum) is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing embryo can survive on its own. The term "egg" is used differently outside the animal kingdom, for an egg cell (sometimes called an ovum). Reproductive structures similar to the egg in other kingdoms are termed spores, or (in spermatophytes) seeds.

Oviparous animals are animals that lay eggs, with little or no other development within the mother. The study or collecting of eggs, particularly bird eggs, is called oology.

Reptile eggs, bird eggs, and monotreme eggs, which are laid out of water, are surrounded by a protective shell, either flexible or inflexible. The special membranes that support these eggs are traits of all amniotes, including mammals.