Definition: A sperm cell swims toward an ovum cell (egg) and fertilises it. The sperm cell is male and the ovum cell (egg) is female. When the man ejaculates, the sperm produced swims along until it meets the female sex cell (egg). When the sperm cell and the egg meet, the single sperm cell's head bores through the wall of the egg and fertilises it. The fertilised cell then starts dividing and getting bigger, becoming an embryo.
reproduced by permission from Hodder