A dehumidifier is an apparatus far removing water vapor from the air. A room dehumidifier is a small, portable unit that can be placed in the center of a damp room to dry the air. Most dehumidifiers operate by passing the moist air through a cooling device similar to the cooling coil of a refrigerator.
Cooling makes the water vapor condense out of the damp air. The condensed vapor collects in a pan, which must be emptied periodically.