Architecture. a recession of the upper part of a building from the building line, as to lighten the structure or to permit a desired amount of light and air to reach ground level at the foot of the building.
an act or instance of setting back:
A nightly setback of your home thermostats can save a great deal of fuel.
Also, set-back. a downward temperature adjustment of a thermostat, especially performed automatically, as by a timer.
also set-back, 1670s, "reversal, check to progress," from set (v.) + back (adv.). Sometimes backset was used in the same sense. Meaning "space between a building and a property line" is from 1916. To set (someone) back "cost" is from 1900.