1. Defined by W. Köppen (1931) as the zone having a definite winter with snow, and a short summer, generally hot. It includes a large part of North America between the Arctic Zone and about 40°N, extending to 35°N in the interior. In Central Europe and in Asia the boreal zone extends southward from the tundra to 40°–50°N. 2. A biogeographical zone or region characterized by a northern type of fauna or flora. The term boreal region is used mainly by American biologists, and includes the area between the mean summer isotherm of 18°C or 64. 4°F (roughly 45°N latitude) and the Arctic Zone.