Works produced in the
youth or
childhood of an
artist. These need not be much inferior to their great works or
mature works. They don't have to be "
juvenile" in the sense of
immature.
Mozart wrote many symphonies and some operas when he was a boy, and Mendelssohn and many others also wrote high-quality works when young.
Jane Austen's juvenilia are often hilariously funny, and wicked parodies of fashionable styles in writing. (Go to her node and go through the list: I've now noded most of them, many of them with full texts, as they're short.)
Charlotte Bronte's juvenilia included elaborate stories of the fantasy world of Angria, and her sisters did similar with Gondal.
Most great artists have had some good juvenilia.