Thermometers measure temperature or a temperature gradient. A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb on a mercury-in-glass thermometer) in which some physical change occurs with temperature, and (2) some means of converting this physical change into a numerical value (e.g. the visible scale that is marked on a mercury-in-glass thermometer).
Thermometers are widely used in the health and medical industry to control and regulate processes, in the study of weather, in medicine, and scientific research.