a support that consists of a horizontal surface for holding objects
"The heavy box slipped off the wooden shelf and shattered on the floor below."
A flat, rigid structure, fixed at right angles to a wall or forming a part of a cabinet, desk etc., and used to support, store or display objects.
"He carefully arranged the antique books on the new oak shelf he installed above the fireplace."
In plain English: A shelf is a flat board attached to a wall or inside furniture that you use to hold things.
"I put my books on the top shelf."
Usage: Use the singular form when referring to one horizontal surface within furniture or on a bookcase. The plural "shelves" is required if you are describing multiple levels for storing items like books or dishes.
The word shelf comes from the Old English sċylfe, originally meaning a ledge or the deck of a ship. It traces back to a root related to cutting, which is also found in words like sculpt and shell.