biweekly / bimonthly
Twice in a period versus once every two periods.
Biweekly can mean twice a week or every two weeks. Bimonthly can mean twice a month or every two months. Both are ambiguous. Use twice a week, every two weeks, twice a month, or every two months.
Examples
Use The report is due every two weeks.
Avoid The report is due biweekly.