What is the name given to the diploid cell formed when a sperm fertilizes an ovum?

The diploid cell formed when a sperm fertilizes an ovum is known as a zygote. This process marks the beginning of a new organism's development, as the zygote is formed by the fusion of the genetic material from both the sperm and the ovum, which each contain haploid sets of chromosomes. When they combine, they restore the diploid number, providing a complete set of genetic information.

The zygote undergoes several rounds of cell division (mitosis) and begins the process of development, eventually forming an embryo. However, at the initial stage when the sperm and ovum fuse, the correct term is zygote, distinguishing it from later developmental stages such as the embryo, which refers to a more developed organism that follows the zygote stage.

The terms blastocyst and gamete do not apply here; a blastocyst refers to a specific stage of development that occurs after the zygote has divided and formed a hollow ball of cells, while gametes are the reproductive cells (sperm and ova) that combine during fertilization. Thus, zygote accurately describes the initial diploid cell resulting from fertilization.