From AMS Glossary
In C. W. Thornthwaite's 1948 climatic classification, any type of climate in which the seasonal water surplus counteracts seasonal water deficiency; thus it has a moisture index greater than zero. Included are the moist subhumid, humid, and perhumid climates. In contrast are the dry climates. The moist climates are further subdivided according to values of aridity index into the following: little or no moisture deficiency; moderate summer water deficiency; moderate winter water deficiency; large summer water deficiency; and large winter water deficiency.
Compare rainy climate.
Thornthwaite, C. W. 1948. An approach toward a rational classification of climate. Geogr. Rev.. 38. 55–94.