What do you mean by Double Fertilization?

The kind of fertilization observed in most of the angiospermic plants where one male gamete (n) fertilizes female gamete (n) and the other male gamete fertilizes the definitive nucleus (2n) at a time is called double fertilization.

In this process, the ovum turns to a zygote (2n) and the definitive nucleus to an endosperm (3n).