What is the X in X-mas?
combining form used in forming the names of devices for stabilizing or regulating (thermostat, etc.), from Greek statos "standing, stationary," from histanai "to cause to stand," from PIE root *sta- "to stand" (see stet). First used in heliostat "an instrument for causing the sun to appear stationary" (1742).
Something that stabilizes: barostat.
Something that inhibits: hemostat.