/set"bak'/ , n.
1. a check to progress; a reverse or defeat: The new law was a setback.
2. Archit. 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.
3. an act or instance of setting back: A nightly setback of your home thermostats can save a great deal of fuel.
4. Also, set-back . a downward temperature adjustment of a thermostat, esp. performed automatically, as by a timer.
[ 1665-75; n. use of v. phrase set back ]