A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in colour. In many organisms the juvenile has a different name from the adult (see also List of animal names).
Some organisms reach sexual maturity in a short metamorphosis, such as eclosion in many insects. For others, the transition from juvenile to fully mature is a more prolonged process — puberty for example. In such cases, juveniles during this transformation are sometimes called subadults.
Many invertebrates, on reaching the adult stage, are fully mature and their development and growth stops. Their juveniles are larvae.
In vertebrates and some invertebrates (e.g. spiders), larval forms (e.g. tadpoles) are usually considered a development stage of their own, and "juvenile" refers to a post-larval stage that is not fully grown and not sexually mature. In amniotes and most plants, the embryo represents the larval stage. Here, a juvenile" is an individual in the time between hatching/birth/germination and reaching maturity.