I was taught that "bimonthly" means every two months and "semimonthly" means twice per month. However, the Miriam Webster site gives two definitions for "bimonthly:
1 : occurring every two months
2 : occurring twice a month : semimonthly
On the same site, you can find "biannual" meaning "semiannual" and the opposite of "biennial."
We have a screwy language.