When the air in a room is too dry, you humidify it: you put very small drops of water into the air so it feels less dry. People usually do this with a machine (a humidifier), especially in winter when heaters dry the air out, or in a place that must stay a little wet, like a room for wooden instruments. You can also humidify a greenhouse, or a box for cigars. The word is a bit technical; at home people more often say they will put the humidifier on than they will humidify the room, though both are correct. The opposite is dehumidify — take water out of the air. You humidify a room or the air; you do not humidify a person.