A zygote is the cell that forms when an egg and a sperm come together [combine] to start the development of a new living being. It is the very first stage of life for humans and many other animals. For example, when the sperm of a man fertilizes a woman’s egg, it creates a zygote that will eventually grow into a baby.
Very Simple Definition:
A zygote is the very beginning of a baby. It happens when a sperm and an egg join together.