What is a zygote? Definition:

When the union of egg and sperm arises with the zygote (Greek zygotos = tied together) a cell with a complete set of chromosomes.
The oocyte and sperm cell have only one simple set of chromosomes with 23 chromosomes each. An odd number of chromosomes in the genome makes normal cell division (mitosis) impossible. If this cell were to divide, one cell would have eleven, and the other cell would have twelve chromosomes. Only with the fusion of the two nuclei (caryogamy) of the gametes there is again a complete, dividable set of chromosomes.
Within the embryogenesis the zygote develops into a morula.