Biannual, biennial, semiannual, though not synonymous, are frequently confused.
The chief source of confusion is biannual, which is used to mean either twice a year or every two years. If no clue to its meaning is given by the text, the reader will often be at a loss to determine the time represented.
Because of this ambiguity there is a tendency to avoid biannual and use the other two terms to express the two meanings.
Biennial unequivocally means existing for two years or happening every two years.
Semiannual is also unequivocal, since it means half-yearly, that is, twice a year.