means a device that automatically regulates temperature by switching heating or cooling on and off to hold a set point.

from Stitched together from Greek parts: thermos, 'hot' (the same root behind 'thermometer' and the vacuum flask), plus -stat, from histanai, 'to make stand' or 'to keep steady' — the same -stat that steadies a 'gyrostat.' So the word literally means 'heat-stander,' a thing that makes the temperature stand still. A 19th-century coinage from the age when inventors loved naming gadgets in tidy Greek.