A young person who is not yet a teenager (being less than thirteen years old), but starting to act like one: generally eight to twelve years old.
1994, Mary Manz Simon, How to Parent Your “Tweenager”, page 6
Your eight to twelve year old is in the gray area between early childhood and the teen years. He is a child in the middle—a “tweenager.”
A child who is not quite old enough to be a teenager; a tweenager.
A shot that is hit between the legs; a tweener.
A 650-watt fresnel motion picture lighting instrument first manufactured by the Mole-Richardson Company.