@zenvelo The prefix is acknowledged to be ambiguous by every authority I’ve checked. This is not a case of one innumerate dictionary. This is not a stupid question.
For entertainment purposes, here is the entry from Fowler’s Modern English Usage, which is a favourite of mine because it is so snarky. This is from the revised edition (1965):
bi- prefixed to English words of time (bi-hourly, bi-weekly, bi-monthly, bi-quarterly, bi-yearly) gives words that have no merits and two faults: they are unsightly hybrids, and they are ambiguous. To judge from the OED, the first means only two-hourly; the second and third mean both two-weekly, two-monthly, and half-weekly, half-monthly; and the last two mean only half-quarterly, half-yearly. Under these desperate circumstances we can never know where we are.
If it were not for bicentenary, which lacks a vernacular equivalent, there would be no reason why all the bi- hybrids should not be allowed to perish, and the natural and unambiguous two-hourly and half-hourly, fortnightly and half-weekly, two-monthly, and half-monthly, half-yearly and half-quarterly, two-yearly and half-yearly, of which several are already common, be used regularly in place of them and the words (biennial, bimestrial) on which they were fashioned; these latter have now almost become ambiguous themselves from the ambiguity of the misshapen brood sprung of them.
They cause confusion in the most surprising places. “An annual bulletin is our first aim; but biennial issues may become possible if the Association enlarges as we hope”. (From a bulletin issued by the International Association of University Professors of English.)
Biannual, probably invented to stand to biennial as half-yearly to two-yearly, is sometimes confused with and sometimes distinguished from it. Half-yearly is the right word.