half-yearly vs biyearly

half-yearly and biyearly both are adjectives.

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

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

Other synonyms of half-yearly include biannual, semiannual.

half-yearly (adjective)	biyearly (adjective)
occurring or payable twice each year	occurring or payable twice each year
	occurring every second year

half-yearly (adverb)	biyearly (adverb)
every half year, every six months	every two years
	twice a year