Which term describes the single cell formed when a sperm cell fertilizes an egg cell?

The term that describes the single cell formed when a sperm cell fertilizes an egg cell is "zygote." This is the initial cell created by the union of a male gamete (sperm) and a female gamete (egg), marking the start of a new organism's development. The zygote contains a full set of chromosomes, half contributed by the sperm and half by the egg, making it diploid (having two sets of chromosomes).

As the zygote undergoes division and development, it will eventually form an embryo, but the embryo is a later stage in development after the zygote has started to divide. Meanwhile, "gametes" refer to the reproductive cells themselves—the sperm and the egg—prior to fertilization. A "blastocyst" is a later structure that develops from the zygote after multiple cell divisions and is involved in implantation into the uterine wall. Each term represents distinct stages or types of cells in the reproductive and developmental process.