What does the term "zygote" refer to in reproductive biology?

The term "zygote" specifically refers to the fertilized egg cell formed when a sperm cell successfully penetrates an egg cell during the process of fertilization. This single cell contains a complete set of chromosomes, half inherited from the mother and half from the father, marking the first stage of development for a new organism.

After the formation of the zygote, it begins to undergo various developmental stages, including cleavage and eventually forming a blastocyst, which is part of early embryonic development. While some options reference other aspects of reproduction, they do not define the zygote itself. For example, the option mentioning a stage in embryonic development pertains to the stages that follow the zygote, rather than defining it directly.