Wet-bulb temperature is the reading a thermometer gives when evaporation is allowed to cool it: the temperature moist air reaches through evaporative cooling. It always lies between the dew point and the dry-bulb temperature, and the three collapse to one value only when the air is already saturated at 100% relative humidity. The contrast with dew point is the useful part — dew point depends only on the air’s actual vapour pressure, the temperature at which cooling alone saturates it, while wet-bulb bakes in the evaporation process itself, via the psychrometer relation.
It is one of the three measurements that can pin down a moist-air state. Given a dry-bulb temperature, supplying any one of relative humidity, wet-bulb, or dew point fixes the state, and every other property — humidity ratio, enthalpy, vapour pressure, specific volume, density — follows from the ASHRAE psychrometrics relations built on the Magnus-Tetens saturation form. For air at 25 °C dry-bulb and 50% RH at sea level, the derived pair reads dew point 13.9 °C and wet-bulb 17.9 °C, matching a standard ASHRAE psychrometric chart at that point.
Those moist-air properties are what the wet-bulb number feeds in practice: cooling-coil loads, dehumidification, and condensation-risk calculations all start from a fixed air state, and altitude matters — saturation pressure depends only on temperature, but humidity ratio, specific volume, and density shift with total pressure, so the same wet-bulb reading describes different air at elevation than at sea level.