What do lengths of time with the "bi" prefix mean"? I have understood bicentennial as once every two hundred years, but biannual as meaning twice a year. Do biweekly and bimonthly mean twice a week or month, or once every two weeks or months?

If this prefix is not used consistently, is there any rationale as to why it isn't?


7 Answers 7


Bi- usually means "every two" (as in "every two [weeks/months/years, etc.]." But it can also mean "twice every [week/month/year, etc.]." To avoid confusion, I save bi- for "every two" and use semi- to mean "twice every." I wish everyone did.

  • 7
    Surely, to avoid confusion, one is sadly forced to bow to peer pressure and use every two for "bi"? Commented Sep 20, 2012 at 10:27

The meanings of the prefix bi are:

  • Occurring twice in every one: biannual, bimonthly.
  • Occurring once in every two: bicentennial, biennial.
  • Lasting for two: biennial, biennium.

The meaning of biannual, for example, could be either the first or the second.

  • 4
    It looks clear and concise, but is misleading. biannual, for instance, should be shown to be in both [your] categories 'a' and 'b'. Commented Sep 20, 2012 at 10:07

In a question which was closed as a duplicate of this question, Steward Godwin Jornsen asked:

I've had some trouble coming up with a one word adjective for 6 months. ... I've thought of semi-annually and bi-annually ... Could there be a better word for it?

While either of the adverbs semiannually (“In a semiannual manner, twice yearly”) or biannually (“Twice per year”) might serve, you could also consider twice-yearly and half-yearly, which fairly transparently mean twice a year and every six months, respectively.

  • Thanks, jwpat7. Voted you up for the good thoughts. That could do, I really need a one worder for 6 months (half-a-year) like: daily, weekly, monthly, quarterly, 6monthly, yearly. Thanks Commented Mar 2, 2013 at 22:04

biweekly = every two weeks
bimonthly = every two months
biennially = every two years

semiweekly = twice a week
semimonthly = twice a month
semiannually = biannually = twice a year

  • Then what are the words for twice a week, month, year, or century? Commented Mar 29, 2011 at 20:44
  • 7
    This answer is technically incorrect. Biweekly can, and often does, mean twice a week. Bimonthly also has a similar double-meaning.
    – MrHen
    Commented Mar 29, 2011 at 20:51
  • 7
    @MrHen is right. Also, I'd prefer biannual to semiannual, although both are equally valid. Putting on my British hat, I'd certainly use fortnightly to mean every two weeks, instead of biweekly, in order to prevent any ambiguity.
    – Jimi Oke
    Commented Mar 29, 2011 at 22:37
  • Wearing British hats can also give rise to ambiguity. My wife has two monthly pensions. One she receives every calendar month, the other bifortnightly. However, I would go with Collins' putting the calendar month as the principal polyseme rather than the AHDEL's attempt to be all-inclusive resulting in the loose: 1 A unit of time corresponding approximately to one cycle of the moon's phases, or about 30 days or 4 weeks. Commented Sep 20, 2012 at 10:20
  • Answer is technically, etymologically, & logically correct. Commenter's objections are based on usage, which could be mentioned in answer. Commented Jul 15, 2023 at 14:38

From Handbook of Technical Writing, 5th Edition (Brusaw, Alred, Oliu):

When used with periods of time, bi means "two" or "every two". Bimonthly means "once in two months"....When used with periods of time, semi means "half of" or "occurring twice within a period of time." Semimonthly means "twice a month".

I thought there was a note that said, "avoid this word because it is confusing," but that is not the case.


The annual values were disaggregated into biannual values. Then the ... (proscribed) Occurring every two years; biennial. 1993, World ... is unclear. Semiannual can be used to mean "every six months", and biennial to mean "every two years"


This is why scientists were tasked to put a man on the moon and not English teachers. Can you imagine if the definition of the rocket coordinates had dual meanings and changed arbitrarily on the whim of a user?

The prefix "bi-" comes from the Latin "bis" meaning "twice". Substituting for "bi-" we have: bi-weekly = twice-weekly; bi-annually = twice annually.

Time for all of the English gurus to get together and settle this once and for all!

  • English is horrible: "biweekly" much more commonly means "every two weeks" while "biannually" much more commonly means "twice a year", yet it is somewhat acceptable to swap them. It's best to avoid these terms. Happily, "semiweekly" and "semiannually" both always mean twice a week/year, so by your usage, you should always favor semi- over bi-.
    – Adam Katz
    Commented Jun 13, 2023 at 3:50

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