occurring twice a year; semiannual.
occurring every two years; biennial.
Let me preface this by saying when we were discussing how to keep your language when you aren't able to speak to people regularly, I always think of soccer as the way i keep up. Soccer forces me to watch German language soccer programs and keep up with it on websites like www.bild.de and www.bundesliga.de.
That being said, I immediately had something in mind when I heard the word biannual. I know it seems that all my posts have to do with soccer but still. Germany's Bundesliga plays on a single table which means there are no conferences and all teams play each other twice, unlike the NFL where teams only play half the league. So back to the word. The teams biannually...or twice a year. There is what's known as a Hinrunde and Ruckerunde, essentially front round and return round, or back round. This is the same as Mexico's Apertura and Clausura league structure in FML, except there is no trophy for winning the Hinrunde or Rueckrunde, just a total one. There is a winter break between the two rounds lasting from early december to mid February. For example if Hertha Berlin played Bayern Muenchen (the kings of German fussball) at home in the Hinrunde, they would play in Muenchen in the rueckrunde. They play twice each year, making it a biannual event.