Source : Webster's Revised Unabridged Dictionary (1913)

Gyrfalcon \Gyr"fal`con\, n. [OE. gerfaucon, OF. gerfaucon, LL.
   gyrofalco, perh. fr. L. gyrus circle + falco falcon, and
   named from its circling flight; or cf. E. gier-eagle. See
   {Gyre}, n., {Falcon}.] (Zo["o]l.)
   One of several species and varieties of large Arctic falcons,
   esp. {Falco rusticolus} and the white species {F.
   Islandicus}, both of which are circumpolar. The black and the
   gray are varieties of the former. See Illust. of {Accipiter}.
   [Written also {gerfalcon}, {gierfalcon}, and {jerfalcon}.]

Source : WordNet®

gyrfalcon
     n : large and rare arctic falcon having white and dark color
         phases [syn: {gerfalcon}, {Falco rusticolus}]