Thermometer
An instrument that gives temperature a number by letting a measurable physical property come to equilibrium with the thing measured.
Definition
A thermometer is any device that converts temperature into a readable quantity by exploiting a physical property that varies reproducibly with it — the volume of a liquid, the pressure of a gas, the resistance of a wire, or the radiation emitted by a hot surface. It works because of thermal equilibrium: the thermometer is a small body allowed to reach the same temperature as its subject, so its property reports the subject's temperature by proxy.
Galileo's thermoscope (c. 1592) was the first such instrument, but it had no fixed scale and responded to air pressure as well as heat. The quantitative thermometer arrived when Fahrenheit sealed mercury into a glass tube in 1714, removing the pressure dependence and anchoring the scale to reproducible fixed points. Modern thermometry spans mercury and alcohol thermometers, platinum-resistance and thermocouple sensors, and infrared pyrometers.
The two requirements of a good thermometer are a property that varies smoothly and monotonically with temperature, and fixed points that can be reproduced anywhere — historically the freezing and boiling of water, and now the triple point of water and other defined standards.
History
From Galileo's open thermoscope to Fahrenheit's sealed mercury instrument (1714) and Celsius's calibrated scale (1742); the triple point of water defined the kelvin from 1954 until the 2019 SI redefinition in terms of the Boltzmann constant.