"Biweekly", "bimonthly", "biannual", and "bicentennial"

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.

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.

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.