English 'thermometer' from Greek 'thermós' (warm) + 'métron' (measure), from PIE *gʷʰer- (warm) — a cousin of English 'warm' and 'furnace.'
An instrument for measuring temperature, typically consisting of a graduated glass tube containing a liquid that expands when heated.
A learned compound of two Greek elements coined in the early 17th century: 'thermós' (θερμός, warm, hot) + 'métron' (μέτρον, a measure, a standard of measurement). 'Thermós' derives from PIE *gʷʰer- (warm, hot), a root that also underlies Latin 'formus' (warm), Old English 'wearm' (warm, from PIE *gʷʰormo-), and English 'furnace' (via Latin 'fornax,' an oven). The second element 'métron' comes from PIE *meh₁- (to measure), also the source of Latin
The word 'thermometer' combines Greek roots meaning 'heat-measure,' but the first practical thermometers were invented by Galileo (c. 1592) and Santorio Santorio (1612) before the word existed. Three major temperature scales were each named after their inventors: Fahrenheit (1724, by Daniel Fahrenheit), Celsius (1742, by Anders Celsius, who originally had the scale inverted — 0° was boiling), and Kelvin (1848, by Lord Kelvin, starting from absolute zero).