A zygote is the result of the union of an ovum (egg cell) and a sperm cell. It remains a zygote until it begins to divide; at that point, the zygote becomes an embryo.
In multi-cellular organisms, a zygote is the first stage of development following conception. Unlike the ovum and sperm, which each contain half the number of chromosomes found in somatic cells, the zygote contains the full number of chromosomes normal in the somatic cells of the species. A zygote normally develops into a single embryo. If the daughter cells resulting from a zygote's division are split apart, each cell can develop into an embryo, resulting in monozygotic twins.