Source : Webster's Revised Unabridged Dictionary (1913)

Nivose \Ni`vose"\, n. [F., fr. L. nix. nivis, snow.]
   The fourth month of the French republican calendar
   [1792-1806]. It commenced December 21, and ended January 19.
   See {Vend['E]miaire}.

Source : WordNet®

Nivose
     n : fourth month of the Revolutionary calendar (December and
         January); the snowy month