The process of forming a zygote, the union of gametes, is called

The process of forming a zygote, the union of gametes, is called fertilization.

Explanation:

Fertilization is the fusion of a male gamete with a female gamete in order to produce a zygote. The male gamete donate the sperm cells while the female gamete fertilizes it to produce a zygote.

'Gametes are formed through a process of cell division called meiosis. This two-step division process produces four haploid daughter cells. Haploid cells contain only one set of chromosomes. When the haploid male and female gametes unite in a process called fertilization, they form what is called a zygote.'