calendar year vs twelvemonth

calendar year and twelvemonth both are nouns.

Word	Noun	Adjective	Verb	Adverb
calendar year	Yes	No	No	No
twelvemonth	Yes	No	No	No

As nouns, twelvemonth is a hypernym of calendar year; that is, twelvemonth is a word with a broader meaning than calendar year:

calendar year: the year (reckoned from January 1 to December 31) according to Gregorian calendar
twelvemonth: a period of time containing 365 (or 366) days

Other hypernyms of calendar year include year, yr.

calendar year (noun)	twelvemonth (noun)
the year (reckoned from January 1 to December 31) according to Gregorian calendar	a period of time containing 365 (or 366) days