From Glossary of Meteorology
In W. Köppen's 1936 climatic classification, any climate type other than the dry climates. However, it is generally understood that this refers principally to the tree climates and not the polar climates.
Köppen, W. P., and R. Geiger 1930–1939. Handbuch der Klimatologie. Berlin: Gebruder Borntraeger, 6 vols..