Biennially
The Century Dictionary and Cyclopedia
Once in two years; at the return of two years.
the GNU version of the Collaborative International Dictionary of English
adverb.
Once in two years.
Wiktionary, Creative Commons Attribution/Share-Alike License
adverb.
Taking place every two years
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adverb.
every two years
Word Usage
"The prize, given biennially, is for the 2008 award."
cross-reference
biannually