Humidity is the dampness or water in the air.
Humidity is one of those things that is usually best in moderation. Too much and things are damaged by the damp -- mold grows, wallpaper peals. Too much humidity in cold weather can result in condensation and damage to exterior walls and windows. Not enough causes other kinds of damage like warping wood or health issues from too dry sinuses, etc. Dry air results in static electricity, dry skin, hair and mucus membranes.
Warm air can hold more humidity than cooler air. (That's why as the air cools down at night, it sometimes releases dew or frost.)
- Piano Technicians Guild on Humidity Control
- HowStuffWorks: What is relative humidity and how does it affect how I feel outside?
- Wikipedia's article on humidity
- Minnesota Blue Flame Gas Association Humidity and the Indoor Environment