civil year vs twelvemonth

civil year and twelvemonth both are nouns.

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

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

civil 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 civil year include year, yr.

civil 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