Learner's definition of SHELF
a flat board which is attached to a wall, frame, etc., and on which objects can be placed
see also bookshelf
Put the vase on the shelf.
the top/bottom shelf of the kitchen cabinet
Will you get that book down off the shelf?
We don't have much shelf space.
a flat area of rock, sand, etc., especially underwater
see also continental shelf
off the shelf
directly from a store without having to be specially made or ordered
on the shelf
in an inactive state
not active or being used