Why is the process of fertilization in a flowering plant referred to as double fertilization ?

Explanation

For fertilization in angiosperm, one male gamete of pollen tube fuses with egg cell (female gamete) to form zygote. This fusion is called syngamy or generative fertilisation.
Second male gamete from pollen tube fuses with secondary nucleus to form primary endosperm nucleus (3n). This is called Triple fusion. or vegetative fertilisation. Both these together constitute double fertilisation in Angiosperms.