From Academic Kids
A zygote (Greek: ζυγωτόν) is a cell that is the result of fertilization. That is, two haploid cells—usually (but not always) a sperm cell from a male and an ovum from a female—merge into a single diploid cell called the zygote (or zygocyte).
Animal zygotes undergo mitotic cell divisions to become an embryo. Other organisms may undergo meiotic cell division at this time (for more information refer to biological life cycles).
Twins and multiple births can be monozygotic (identical) or dizygotic (fraternal).