twelvemonth(n = noun.time) year, yr - a period of time containing 365 (or 366) days; "she is 4 years old"; "in the year 1920"
is a kind of period, period of time, time period
is a part of decade, decennary, decennium
has particulars: christian year, church year, holy year, new year, 365 days, common year, 366 days, bissextile year, intercalary year, leap year, off year, off year, calendar year, civil year, financial year, fiscal year, annum, year of grace, y2k

   A year which consists of twelve calendar months. [1913 Webster]
"I shall laugh at this a twelvemonth hence." [1913 Webster]

twelvemonth, n. archaic a year; a period of twelve months.