BIANNUAL means “twice a year,” or two times within a 365-day period. The prefix “bi-” means “two” and “annual” means “year.”

If your HVAC company has a biannual maintenance plan, they’ll come out two times a year and check your heating and cooling system. Thus, you’ll receive that service twice within a calendar year. Likewise, you should check your smoke detector batteries on a biannual basis.

BIENNIAL, on the other hand, means “once every two years” OR “exists or lasts for two years.” It is derived from the Latin “biennium,” meaning “two-year period.”

Therefore, if you have a contract with a chimney cleaning company that offers biennial service, you’ll only have cleanings once every other year. That means if you get your chimney cleaned on March 6, 2019, you will not have them cleaned again until March 6, 2021.

You’ve probably also heard the term “biennials” in reference to plants (along with annuals and perennials). That means it takes two years to complete its biological plant life cycle. Foxglove, black-eyed Susans, and Sweet William are some common biennial plants.

NOTE: There’s another word, “semiannual,” which is often used interchangeably with “biannual.”

Semiannual means occurring twice a year; the adverb form is semiannually. Semiannual is derived from the prefix semi-, which means half, twice, as it comes from the Latin semi-, meaning half. With “semiannual” the emphasis on “half” is the slight difference, as in it occurs every six months. For instance, a company may publish its semiannual earnings report in January and July (6 months apart, or every half-year). This is different from biannual, which just means occurring two time in a year.

Since semiannual things also occur two times a year, semiannual gets interchanged with biannual. It seems nowadays that semi- has taken on the meaning of “part” as well as “half” so the argument for semiannual to be interchangeable with biannual has some backing. In fact, many people use “seminannual” instead of “biannual” specifically to avoid the biannual/biennial confusion. This is considered acceptable.

Nevertheless, each word has its own distinct meaning:

BIANNUAL: occurring twice a year
BIENNIAL : occurring once every two years/lasting for a two-year period
SEMIANNUAL: occurring half-yearly (or, more recently, twice a year)

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