The definition of a thermometer is a tool used to measure temperature.
An example of a thermometer is a little stick with mercury in it that tells you how hot it is outside or how high your fever is.
thermometer definition by Webster's New World
an instrument for measuring temperatures, consisting of a graduated glass tube with a sealed, capillary bore in which mercury, colored alcohol, etc. rises or falls as it expands or contracts from changes in temperature
any similar instrument, as one operating by means of a thermocouple
Origin: French thermomètre: see thermo- and amp; -meter
thermometer definition by American Heritage Dictionary
An instrument for measuring temperature, especially one having a graduated glass tube with a bulb containing a liquid, typically mercury or colored alcohol, that expands and rises in the tube as the temperature increases.
An instrument used to measure temperature. There are many types of thermometers; the most common consist of a closed, graduated glass tube in which a liquid expands or contracts as the temperature increases or decreases. Other types of thermometers work by detecting changes in the volume or pressure of an enclosed gas or by registering thermoelectric changes in a conductor (such as a thermistor or thermocouple).