Bimonthly – Thai lottery draw is operated bimonthly, for sure. But the meaning of ‘bimonthly’ (also spelled bi-monthly) is not as clear as it looks. This word has a double meaning; it means either ‘once every two months’ or ‘twice a month’.
The case of ‘biweekly’ is similar, meaning either ‘once every two weeks’ or ‘twice a week’. The word ‘fortnightly’(once every two weeks) and the American English word ‘semimonthly’(twice a month) and ‘semiweekly’(twice a week) help get rid of the confusion, but apart from ‘fortnightly’ in British English are less popularly used.
While listening or reading, we can do nothing but be careful in guessing the true meaning from the context. For writing or speaking, we can add more words/phrases to help clarify what exactly is meant.
(In the case of year, ‘biannual’(twice a year) and ‘biennial’(every two years) are not confusing, except for their close spelling.)