A thermometer is a device that measures temperature or a temperature gradient (how hot or cold an object is). A thermometer has two important parts:
A temperature sensor (for example the bulb of a mercury-in-glass thermometer or the pyrometer sensor of an infrared thermometer) in which a change occurs with a change in temperature.
A way to convert that change to a numerical value (e.g. the visible scale that is marked on a mercury-in-glass thermometer or digital readout on an infrared model).
Thermometers are widely used in technology and industry to monitor processes, in meteorology, in the food industry, in medicine and in scientific research.
Some of the principles of the thermometer were known to Greek philosophers two thousand years ago. The Italian physician Santorio Santorio (Sanctorius, 1561-1636) is generally credited with the invention of the first thermometer, but its standardization was completed during the 17th and 18th centuries. Daniel Gabriel Fahrenheit invented the mercury-in-glass thermometer (the first widely used, accurate, and practical thermometer) and the Fahrenheit scale (the first standardized temperature scale to be widely used).