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  1. 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 with a calibrated scale marked on the tube or its frame.

Origin of thermometer

First recorded in 1615–25; thermo- + -meter
Related formsther·mo·met·ric [thur-muh-me-trik] /ˌθɜr məˈmɛ trɪk/, ther·mo·met·ri·cal, adjectivether·mo·met·ri·cal·ly, adverb
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for thermometer


  1. 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 tubeSee also clinical thermometer, gas thermometer, resistance thermometer, thermocouple, pyrometer
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for thermometer


1630s, from French thermomètre (1620s), coined by Jesuit Father Leuréchon from Greek thermos "hot" (see thermal) + metron "measure" (see -meter). An earlier, Latinate form was thermoscopium (1610s). The earliest such device was Galileo's air-thermometer, invented c.1597.

Online Etymology Dictionary, © 2010 Douglas Harper

thermometer in Medicine


  1. An instrument for measuring temperature.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

thermometer in Science


  1. 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).
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.