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 eventually 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 term "conception" commonly refers to fertilisation, but is sometimes defined as implantation or even "the point at which human life begins," and is thus a subject of semantic arguments about the beginning of pregnancy, within the abortion debate. Gastrulation is the point in development when the implanted blastocyst develops three germ layers, the endoderm, the exoderm and the mesoderm. It is at this point that the genetic code of the father becomes fully involved in the development of the embryo. Until this point in development, twinning is possible. Additionally, interspecies hybrids which have no chance of development survive until gastrulation. However this stance is not entirely warranted since human developmental biology literature refers to the "conceptus" and the medical literature refers to the "products of conception" as the post-implantation embryo and its surrounding membranes. The term "conception" is not usually used in scientific literature because of its variable definition and connotation.