Fertilisation or fertilization also known as conception, fecundation and syngamy, is fusion of gametes to form a new organism of the same species. In animals, the process involves a sperm fusing with an ovum, which finally leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation.

The entire process of development of new individuals is called procreation, the act of species reproduction.