This entire phenomenon of fertilisation involving the fusion of one male gamete with the egg, together with the union of the second male gamete with the secondary nucleus or the polar nuclei is called as double fertilisation.
Syngamy can be defined as the process of fusion of egg nucleus with male gamete to form a diploidzygote.
Triple fusion can be defined as the fusion involving two polar nuclei and a sperm nucleus that occurs in double fertilization in a seed plant, giving rise toa triploid nucleus called the primary endosperm nucleus, which later develops into theendosperm.