A thermometer is a device that measures temperature or a temperature gradient (the degree of hotness or coldness of an object) A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermometer or the pyrometric sensor in an infrared thermometer) in which some change occurs with a change in temperature; and (2) some means of converting this change into a numerical value (e.g. the visible scale that is marked on a mercury-in-glass thermometer or the digital readout on an infrared model). Thermometers are widely used in technology and industry to monitor processes, in meteorology, in medicine, and in scientific research. During the COVID-19 pandemic they were used by businesses to detect the fever brought on by the virus.

Some of the principles of the thermometer were known to Greek philosophers of two thousand years ago. The Italian physician Santorio Santorio (Sanctorius, 1561-1636) is commonly credited with the invention of the first thermometer, but its standardisation was completed through the 17th and 18th centuries.

There are many types of thermometers used under Science, Geography etc.