Source : Webster's Revised Unabridged Dictionary (1913)

Boreal \Bo"re*al\, a. (Biogeography)
   Designating or pertaining to a terrestrial division
   consisting of the northern and mountainous parts of both the
   Old and the New World; -- equivalent to the Holarctic region
   exclusive of the Transition, Sonoran, and corresponding
   areas. The term is used by American authors and applied by
   them chiefly to the Nearctic subregion. The Boreal region
   includes approximately all of North and Central America in
   which the mean temperature of the hottest season does not
   exceed 18[deg] C. (= 64.4[deg] F.). Its subdivisions are the
   Arctic zone and

   {Boreal zone}, the latter including the area between the
      Arctic and Transition zones.