What is the term for a fertilized egg formed by the union of sperm and egg?

The correct term for a fertilized egg formed by the union of sperm and egg is zygote. This stage represents the very first moment of development after fertilization, where the genetic material from both the sperm and the egg combine to create a single cell. This zygote undergoes multiple rounds of cell division and eventually develops into an embryo as it continues to grow and differentiate.

An embryo refers to the later stage of development following the zygote, which encompasses the stages during which significant cellular differentiation and organ formation occur. The term gamete specifically refers to the reproductive cells themselves—sperm in males and eggs in females—rather than the fertilized result of their combination. Lastly, the term cell is a more general designation that refers to the smallest structural and functional unit of an organism, which can include various cell types beyond just the zygote.