1. Something forming a back. 2. The wood blocking behind plastering for supporting the load of lighting fixtures, cabinets, and hardware attached to a wall or ceiling. 3. Any material used as a base over which a finished material is to be installed. 4. See Plaster Base. 5. Material (metal, weld metal, asbestos, carbon, granular flux, etc.) backing up the joint during welding to facilitate obtaining a sound weld at the root. 6. The carpet foundation of jute, kraftcord, cotton, rayon, or polypropylene yarn that secures the pile yarns and provides stiffness, strength and dimensional stability. 7. The part of the wall behind the face brick.