A concept from evolutionary biology: basically that sometimes organisms adapt by not fully maturing or that evolution makes use of what was an incompletely developed phase in an evolutionary ancestor's development as the adult form of a new organism.
Examples include the axlotl, a Mexican salamander which (under the pressures of a hot, arid terrestrial environment) stays in a "tadpole" like stage, remaining fully aquatic throughout its life cycle. Also, human evolutionary development is said to example neoteny. Higher primates, and in fact many simians less intimately related to humans genetically, have young whose still not fully developed and specialized forms are more strikingly human-like than their adult counterparts, having juvenile characteristics such as larger (proportionally) eyes, flatter faces with less snout-like protrusiveness, and rounder heads and faces generally. Human evolutionary development is thought to have adopted these forms as the traits of adult organisms, lumped in along with the prolonging of infancy and childhood which allows for the much heightened demands of human mental development.
The term is also evoked in discussion of depictions in art, notably the anthropomorphic, neotenic forms of cartoon animals and animated characters in advertisements, where the infantile characteristics (particularly the large, round eyes and proportionally shortened arms and legs) are exaggerated to evoke an instinctive emotional response from human viewers who subconsciously associate exaggerated neoteny with human infants, to whom they are instinctively hardwired to pay attention. Much anime also makes use of this formula, see kawaii.