relative humidity

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The ratio of the actual amount of water vapor (absolute humidity) present in the air to the saturation point at the same temperature, usually expressed as a percentage.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the ratio of the amount of water in the air at a give temperature to the maximum amount it could hold at that temperature; expressed as a percentage