The heat that is lost (in BTU's) through ceilings, roof, floors and exterior walls of a house.
Heat that is lost from a building through air leakage, conduction and radiation. To maintain a steady interior temperature, heat losses must be offset by a combination of heat gains and heat contributed by a heating system.
The amount of heat that leaves the living space, as through walls, ceiling, air leakage around windows, etc. This is the amount of heat that a furnace has to replace in order to maintain a constant temperature in the living space.