backup 1. That part of a masonry wall behind the exterior facing.
2. A compressible material used behind a sealant to reduce its depth and to support the sealant against sag or indentation.
3. Overflow in a drain or piping system, due to stoppage.
4. A condition where waste water flows back into another fixture or compartment or water line (but does not flow back into the
potable water system).