From AMS Glossary
The degree to which a climate lacks effective, life-promoting moisture; the opposite of humidity, in the climate sense of the term.
The overall concept of aridity versus humidity is coming to be known as precipitation effectiveness. Two basic approaches have been made. The first, used by W. Köppen and modified by Bailey, does not openly define aridity, but rather assigns delimiting values of annual precipitation (treated with regard to distribution and temperature) to separate a dry climate from other types. The second approach actually prescribes a measure of aridity or precipitation effectiveness and uses these values as a primary parameter of classification. Of this type are Thornthwaite's precipitation- effectiveness index and moisture index, E. de Martonne's index of aridity, W. Gorczyński's aridity coefficient, Lang's moisture factor, and Ångström's humidity coefficient.