From Ancient Greek ὤν, Latin on > gen. ὄντος, ontos "being" + γένεσις, genesis "origin".

Ontogeny is the process of quantitative and qualitative changes occurring in an organism from the moment of fertilization of the egg until the death of the adult.

A set of successive morphological and biochemical transformations undergone by an organism from fertilization (in sexual reproduction) or from the moment of separation from the mother (in sexual reproduction) to the end of life.

The term "ontogeny" was first introduced by E. Heckel in 1866. In the course of ontogeny there is a process of realization of genetic information received from parents.

The section of modern biology that studies ontogeny is called developmental biology; the initial stages of ontogeny, embryogenesis, are also studied by embryology.