The process of fusion of one male gamete with the egg along with the union of the second male gamete with the two polar nuclei or the secondary nucleus is called double fertilisation.
Out of the two male gametes, one fuses with the egg to carry out generative fertilisation or syngamy. It gives rise to a diploid zygote or oosphere.
The nucleus of the second male gamete fuses with two haploid polar nuclei or diploid secondary nucleus of the central cell to form a triploid primary endosperm nucleus (PEN). The central cell is called primary endosperm cell (PEC). This is called vegetative fertilisation or triple fusion.
