Neoteny (/niːˈɒtɨni/ /niːˈɒtni/ or /niːˈɒtəni/), also called juvenilization, is one of the three ways by which paedomorphism can arise. Paedomorphism or paedomorphosis is the retention by adults of traits previously seen only in the young, and is a subject studied in the field of developmental biology. In neoteny, the physiological (or somatic) development of an organism (typically an animal) is slowed or delayed. In contrast, in progenesis, sexual development occurs faster. Both processes result in paedomorphism. Ultimately this process results in the retention, in the adults of a species, of juvenile physical characteristics well into maturity and pedogenesis (paedogenesis), the reproduction in a neotenized state.
Neoteny is one of three dimensions of heterochrony, or the change in timing of developmental events: acceleration (faster) vs. neoteny (slower), hypermorphosis (further) vs. progenesis (not as far), and predisplacement (begins earlier) vs. postdisplacement (begins later).
The word neoteny is borrowed from the German Neotenie, the latter constructed from the Greek νέος (neos, “young”) and τείνειν (teínein, “to extend”). The adjective form of the word is either “neotenic” or “neotenous”. For the opposite of “neotenic”, different authorities use either “gerontomorphic” or “peramorphic”.