biannual / biennial / semiannual
Twice a year versus every two years.
Biennial means every two years. Semiannual means twice a year. Biannual is used both ways and so is a poor choice when the span matters. If the date is the point, use twice a year or every two years.
Examples
UseThe biennial conference meets in even years.
UseThe board holds a semiannual review.
Avoida biannual review (If a reader cannot tell whether you mean twice a year or every two years.)