A zygote is the single-cell that is produced when a sperm fertilizes an egg. It is the first cell in the human body.
FertilitySmarts explains Zygote
When a female sex cell, the egg, and a male sex cell, sperm, meet, the sperm may penetrate the egg’s exterior, causing the mixing of the 23 male and 23 female chromosomes. The now fertilized egg is considered a zygote.
The zygote is covered in a protective covering known as the zona pellucida and contains 46 unique chromosomes. Chromosomes contain tightly packed molecules called DNA that contain all of the instructions needed to transform the single cell into a fetus.
With natural conception, zygote development typically occurs in the fallopian tubes, where it completes the first cell division around 24 to 30 hours after fertilization. After around three days of development, the zygote becomes a solid ball called the morula. By around day four, the morula becomes a blastocyst, the next phase of embryo development.