bi-, semi-
Forms solid compounds except when followed by a capital letter or i, or when unreadable.
To avoid confusion, the following uses are strongly recommended:
semiweekly or better twice a week
biweekly or better every two weeks or every other week
semimonthly or better twice a month
bimonthly or better every two months or every other month
semiannual or better twice a year
biennial or better every two years or every other year