# Biannual vs. semiannual

**URL:** <https://boards.straightdope.com/t/biannual-vs-semiannual/65028>\
**Category:** Factual Questions\
**Created:** [May 10, 2001, 10:28pm UTC](https://boards.straightdope.com/t/biannual-vs-semiannual/65028 "2001-05-10T22:28:14Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![mazzer](https://avatars.discourse-cdn.com/v4/letter/m/eada6e/32.png) [@mazzer](https://boards.straightdope.com/u/mazzer)\
**Post date:** [May 10, 2001, 10:28pm UTC](https://boards.straightdope.com/t/biannual-vs-semiannual/65028/1 "2001-05-10T22:28:14Z")

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Biweekly means once every two weeks.  
Bimonthly means once every two months.  
Biannually means twice a year.

I understand that “annually” is not the same as “yearly,” and thus the departure, but:

The big bad dictionary gives the definition of biannual as: “twice a year; semiannual.” How can these two words mean the same thing? Their prefixes have opposite meanings.

Something strange has happened here. Any help?

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**Author:** ![Chas.E](https://avatars.discourse-cdn.com/v4/letter/c/e99b99/32.png) [@Chas.E](https://boards.straightdope.com/u/Chas.E)\
**Post date:** [May 10, 2001, 10:47pm UTC](https://boards.straightdope.com/t/biannual-vs-semiannual/65028/2 "2001-05-10T22:47:46Z")

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The correct usage would be “biennial.”

Lexicographers make mistakes too.

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**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\
**Post date:** [May 10, 2001, 11:22pm UTC](https://boards.straightdope.com/t/biannual-vs-semiannual/65028/3 "2001-05-10T23:22:20Z")

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> [@](#):
>
> \*Originally posted by chriszarate \*  
> How can these two words mean the same thing? Their prefixes have opposite meanings.

inflammable = flammable 🙂

Regardless of how the parsed-out morphemes should add up, if somebody is using “biannual” to mean “semiannual” then there’s not much you can do about it. The simple fact of the matter is this: a lot more things happen twice a year than happen every two years, especially in nature.

I think the thought process is along these lines, though: if a bicycle is one thing with two parts, then biannual must mean one year divided into two parts. I personally try to avoid all such confusions by using the expression “twice a year.” 🙂

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**Author:** ![BobT](https://avatars.discourse-cdn.com/v4/letter/b/8e8cbc/32.png) [@BobT](https://boards.straightdope.com/u/BobT)\
**Post date:** [May 10, 2001, 11:31pm UTC](https://boards.straightdope.com/t/biannual-vs-semiannual/65028/4 "2001-05-10T23:31:46Z")

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According to the OED “Biannual” as an adjective means “half-yearly” and as a noun it has the same meaning as “Biennial”, which means every two years.

“Semiannual” as both an adjective and a noun means “twice a year”.

“Biennial” is a very old word dating back to the 17th century, but “biannual” didn’t come around until late in the 18th century.

The state of California issues a publication called “Used Oil Recycling Rates”. It is called “A Biannual Report”. And it was released every two years. It is cataloged as “biennial.”

A different office in the same state agency issued something titled “Biannual report on redemption and recycling rates”  
It is cataloged as a “semiannual” and comes out every six months.