The Collaborative International Dictionary of English v.0.48:

Gerfalcon \Ger"fal`con\, n. (Zool.)
   See Gyrfalcon.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gyrfalcon \Gyr"fal`con\ (j[~e]r"f[add]`k'n), 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.] (Zool.)
   One of several species and varieties of large Arctic falcons,
   esp. Falco rusticolus and the white species Falco
   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.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

gerfalcon
    n 1: large and rare Arctic falcon having white and dark color
         phases [syn: gyrfalcon, gerfalcon, Falco rusticolus]