An instrument for measuring temperature: A thermometer is especially one having a graduated glass tube with a bulb containing a liquid, typically mercury or colored alcohol, that expands and rises in the tube as the temperature increases.

In general, substances expand slightly when heated, and shrink when cooled. This fact gave mankind its first tool for measuring temperature accurately, that is the mercury thermometer. Thermometer comes from the Greek words therm, "heat" and metron, "measure"; so, it is an instrument "to measure heat".