what is double fertilisation ? why it is named so?

Double fertilization is unique to the angiosperms. It is the process in which the two sperm nuclei of a pollen grain unite with nuclei of the embryo sac of an angiosperm plant. One sperm nucleus unites with the egg to form the diploid zygote, from which the embryo develops. The other sperm unites with the two polar nuclei located in a single cell at the center of the embryo sac. Since the fusion between sperms and female gamete occurs twice, it is called double fertilisation.