A thermometer is a device that measures temperature. A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb on a spirit-in-glass thermometer，NTC sensor in a digital thermometer) (2) some means of converting this physical change into a numerical value (e.g. the visible scale on dial face of bimetal thermometer).

There are various principles by which different thermometers operate. They include the thermal expansion of solids (bimetal thermometer) or liquids (glass thermometer) with temperature, or the change in pressure of a gas on heating or cooling (remote reading thermometer). Radiation-type thermometers measure the infrared energy emitted by an object, allowing measurement of temperature without contact (infrared thermometer).