Source : Webster's Revised Unabridged Dictionary (1913)

Twelvemonth \Twelve"month\, n.
   A year which consists of twelve calendar months.

         I shall laugh at this a twelvemonth hence. --Shak.

Source : WordNet®

twelvemonth
     n : a period of time containing 365 (or 366) days; "she is 4
         years old"; "in the year 1920" [syn: {year}, {yr}]