Why is fertilization characteristic of angiosperms called double?

This is a reproductive process in angiosperms in which both the egg and the central cell of the embryo sac are fertilized. In double fertilization, both sperm are introduced, introduced into the embryo sac by a pollen tube; the nucleus of one sperm merges with the nucleus of the egg, the nucleus of the second with the polar nuclei or with the secondary nucleus of the embryo sac. An embryo develops from a fertilized egg, and endosperm from a central cell.