Source : Webster's Revised Unabridged Dictionary (1913)

Wet-bulb thermometer \Wet-bulb thermometer\ (Physics)
   That one of the two similar thermometers of a psychrometer
   the bulb of which is moistened; also, the entire instrument.

Source : WordNet®

wet-bulb thermometer
     n : a thermometer with a bulb that is covered with moist muslin;
         used in a psychrometer to measure humidity [ant: {dry-bulb
         thermometer}]