Semi-yearly is a term that refers to something that occurs twice a year. There are several synonyms that can be used to describe this frequency, including biannual, semi-annual, half-yearly, and twice-yearly. Biannual is perhaps the most commonly used synonym, as it is more straightforward than some of the other terms. Semi-annual and half-yearly are also frequently used, though they can be seen as slightly more formal and may be used more in business or academic contexts. Finally, twice-yearly is another option that can be used interchangeably with all of the other synonyms, though it is not as commonly used.