biannual vs half-yearly

biannual and half-yearly both are adjectives.

Word	Noun	Adjective	Verb	Adverb
biannual	No	Yes	No	No
half-yearly	No	Yes	No	Yes

As adjectives, biannual and half-yearly are synonyms defined as:

Other synonyms of biannual include biyearly, semiannual.

biannual (adjective)	half-yearly (adjective)
occurring or payable twice each year	occurring or payable twice each year

biannual (adverb)	half-yearly (adverb)
	every half year, every six months