Biannually is a term used to refer to something that occurs twice a year or every six months. However, it is always good to know some synonyms that can be used interchangeably. Semi-annually is one of the most common synonyms for biannually. Alternatively, you can use twice-yearly or every half-year to mean the same thing. Two per annum and biyearly are other synonyms you can use as well. It is essential to note that any of these terms can be used according to the context they are applied in, and one can use any that they feel comfortable with.