"Zygote" is the scientific term that refers to a fertilized egg cell. That means that it is formed when an egg cell is fertilized by a sperm cell.
In human reproduction, a fertilized egg is referred to as a zygote for roughly the first four days after fertilization. It is called this during the time that it is in the Fallopian Tubes. By the time it reaches the uterus and implants in the uterine lining, it has divided enough times that it consists of about 100 cells. At that point, it stops being called a zygote and is referred to as an embryoblast.
Of the links below, the enotes link has a great written description of what happens, the "daviddarling" has a diagram, and the PBS has a slide show about it. The slide show is the "mitosis vs. meiosis" link on that page.
You can reference the links below for full information
A zygote is formed when an egg is fertilized by a sperm. Cell division then takes place up to about 100 cells, and during the process it is a zygote.