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?
The correct usage would be “biennial.”
Lexicographers make mistakes too.
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.”
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.