A hygrometer consisting of a dry-bulb thermometer and a wet-bulb thermometer

• Their difference indicates the dryness of the surrounding air

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"instrument to measure moisture in the atmosphere," 1749, from Latinized form of Greek psykhros "cold" + -meter .

WordNet Word definitions in WordNet
n. a hygrometer consisting of a dry-bulb thermometer and a wet-bulb thermometer; their difference indicates the dryness of the surrounding air

Wiktionary Word definitions in Wiktionary
n. (context meteorology English) Any of several instruments used to measure the relative humidity of the atmosphere; especially a wet-and-dry-bulb hygrometer.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Psychrometer \Psy*chrom"e*ter\, n. [Gr. psychro`s cold + -meter: cf. F. psychrom[`e]tre.] An instrument for measuring the tension of the aqueous vapor in the atmosphere, being essentially a wet and dry bulb hygrometer.