a thermometer the bulb of which is covered by a moist muslin bag, used together with a dry-bulb thermometer to measure humidity
wet-bulb thermometer in American English
(ˈwɛtˈbʌlb)
Meteorology
a thermometer having its bulb covered with a wet cloth: the water evaporates to cool the wet bulb, and by comparing its temperature to a dry-bulb thermometer the amount of water vapor in the air can be determined