ju·ve·nile (jo?o?v?-n?l?, -n?l)adj.1. a. Not fully grown or developed; young.b. Of or characteristic of a young animal that has not reached sexual maturity: a bird still in juvenile plumage.2. Characteristic of, intended for, or appropriate for children or young people: juvenile fashions.3. Marked by immaturity; childish: juvenile behavior. See Synonyms at young.4. Geology Relating to or being water, gas, or a mineral-rich fluid believed to have originated from magma and to have come to the earth’s surface for the first time.n.1. a. A young person; a child.b. A young animal that has not reached sexual maturity.c. A two-year-old racehorse.2. An actor who plays roles of children or young persons.3. A children’s book.[Latin iuven?lis, from iuvenis, young; see yeu- in Indo-European roots.]ju?ve·nile?ly adv.ju?ve·nile?ness n.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.