Antonyms of adj biannual

1 sense of biannual

Sense 1
semiannual, biannual, biyearly, half-yearly -- (occurring or payable twice each year)

INDIRECT (VIA periodic) -> aperiodic, nonperiodic -- (not recurring at regular intervals)

Similarity of adj biannual

1 sense of biannual

Sense 1
semiannual, biannual, biyearly, half-yearly -- (occurring or payable twice each year)
       => periodic (vs. aperiodic), periodical -- (happening or recurring at regular intervals; "the periodic appearance of the seventeen-year locust")