I've always been under the impression that biennially means once every other year and that biannually (like semiannually) means twice in one year. Now, someone just showed me that as per dictionary.com, biannual could mean either of these!
It cites The Random House Dictionary and The American Heritage Dictionary as sources. Have I been wrong all along ?
My Compact OED says that
biennially means once every two years (as you thought) and
biannually means half-yearly (or twice a year - again, as you thought). Those are the only definitions offered.
My dictionary is a 1985 printing, so I wonder if with the dynamic nature of language,
biannual as evolved to mean more than its original meaning?