1.Definition: (of birds) not yet having developed feathers

Related Adjective(s):unfledged

2.Definition: (used of living things especially persons) in an early period of life or development or growth

Related Adjective(s):young

3.Definition: characteristic of a lack of maturity

"Immature behavior"

4.Definition: not fully developed or mature; not ripe

Related Adjective(s):green, unripe, unripened

5.Definition: not yet mature