WordWeb Online

1. Characteristic of a lack of maturity
   "immature behaviour"
    
2. (used of living things especially persons) in an early period of life, development or growth
   "immature people";
   - young
    
3. Not fully developed or mature; not ripe
   "fried immature tomatoes";
   - green, unripe, unripened
    
4. Not yet adult
    
5. (of birds) not yet having developed feathers
   "a small immature sparrow on the window sill";
   - unfledged

See also: adolescent, age, babyish, boyish, boylike, childish, childlike, childly, early, embryologic, embryonal, embryonic, five-year-old, four-year-old, girlish, inceptive, inchoative, infantile, jejune, junior, juvenile, larval, little, matureness, maturity, new, newborn, one-year-old, preadolescent, premature, prepubertal, prepubescent, prepupal, preteen, pubescent, puerile, pupal, puppyish, puppylike, schoolboyish, schoolgirlish, small, teen, teenage, teenaged, tender, three-year-old, two-year-old, unaged, underdeveloped, unfeathered, vernal, wet behind the ears, young, youngish, youthful

Antonym: fledged, mature, old

Encyclopedia: Immature