What is the term for the initial cell formed when two gamete cells unite during sexual reproduction?

The term for the initial cell formed when two gamete cells unite during sexual reproduction is "zygote." This cell is the first stage of development that results from the fertilization of an egg cell by a sperm cell. The zygote contains genetic material from both parents, which begins the process of cellular division and development into a multicellular organism.

As the zygote undergoes mitotic divisions, it eventually develops into an embryo and later into a fetus. The embryo is an earlier developmental stage, and the fetus is a more advanced stage of development that occurs after the embryonic stage. A blastocyst is a structure formed in the early development of mammals, which arises from the zygote, but it is not the initial cell itself. Thus, "zygote" is the most precise term for that first cell created through the fusion of gametes.