The Collaborative International Dictionary of English v.0.48:
Twelvemonth \Twelve"month\, n. A year which consists of twelve calendar months. [1913 Webster] I shall laugh at this a twelvemonth hence. --Shak. [1913 Webster]WordNet (r) 3.0 (2006):
twelvemonth n 1: a period of time containing 365 (or 366) days; "she is 4 years old"; "in the year 1920" [syn: year, twelvemonth, yr]