fertilization

Union of a male and female gamete: an ovum and a sperm (spermatozoon). If coitus has occurred within a few days of discharge of a ovum from an ovary (normally around the middle of the menstrual cycle or about two weeks from the start of the last period), one among the sperm which have survived the hazards of the journey to the Fallopian tube meets the ovum there, and enters it. This event instantly triggers changes in cellular chemistry and physics in ways which, among other things, prevent access of any rival sperm. A new unique individual is formatted from the conjunction of genetic material, and cell division begins.

— Stuart Judge