The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
Nivose n 1: fourth month of the Revolutionary calendar (December and January); the snowy month