Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

psychrometer
     n : a hygrometer consisting of a dry-bulb thermometer and a
         wet-bulb thermometer; their difference indicates the
         dryness of the surrounding air