neoteny
The retention of juvenile characteristics in the adult.
countable, uncountable
Most amphibians are aquatic only while young, but some amphibians with neoteny remain aquatic even as adults.
Where it comes from
From German Neotenie, coined in 1884 by anatomist Julius Kollmann from Ancient Greek νέος (néos, “young”) + τείνειν (teínein, “to extend, spread or tense”).
Adapted from the Wiktionary entry neoteny (CC BY-SA 4.0).