The Collaborative International Dictionary of English v.0.48:

December \De*cem"ber\ (d[-e]*s[e^]m"b[~e]r), prop. n. [F.
   d['e]cembre, from L. December, fr. decem ten; this being the
   tenth month among the early Romans, who began the year in
   March. See Ten.]
   1. The twelfth and last month of the year, containing
      thirty-one days. During this month occurs the winter
      solstice.
      [1913 Webster]

   2. Fig.: With reference to the end of the year and to the
      winter season; as, the December of his life.
      [1913 Webster]

WordNet (r) 3.0 (2006):

December
    n 1: the last (12th) month of the year [syn: December, Dec]