The actual humidity in terms of the water content of air is typically expressed as grams per kilogram or kilogram per kilogram. Humidity is typically expressed as a percentage of how humid it could physically be at a given temperature.
Absolute humidity is the mass of water vapor in the air divided by the mass of dry air. Relative humidity is a more common method of expressing humidity, and it is the current absolute humidity divided by the maximum possible absolute humidity at the current temperature, expressed as a percentage. A relative humidity of 45 percent is the point at which humans tend to be most comfortable.