Real Dictionary

thermometer
countable noun: A thermometer is an instrument for measuring temperature. It usually consists of a narrow glass tube containing a thin column of a liquid which rises and falls as the temperature rises and falls.

thermometer in American English
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
noun: an instrument for measuring temperature, often a sealed glass tube that contains a column of liquid, as mercury, that expands and contracts, or rises and falls, with temperature changes, the temperature being read where the top of the column coincides wit

thermometer in British English
noun: an instrument used to measure temperature, esp one in which a thin column of liquid, such as mercury, expands and contracts within a graduated sealed tube

thermometer in Mechanical Engineering
noun: A thermometer is a device for measuring temperature, usually one that works by the expansion of a column of fluid.