<p><strong>Noun</strong></p><ul><li>a check to progress; a reverse or defeat: The new law was a setback.</li><li>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.</li><li>an act or instance of setting back: A nightly setback of your home thermostats can save a great deal of fuel.</li><li>Also: a downward temperature adjustment of a thermostat, especially performed automatically, as by a timer.</li></ul>