Biannual and Biennial, both adjectives, have meanings you can probably guess.
Biannual means “twice a year” and biennial means “every two years”:
In New Orleans we miss Mardi Gras for most of the year, so we made it a biannual event; we have regular Mardi Gras in February, and Midsummer Mardi Gras in August!
Some academic conferences are biennial—they only happen every other year.
Some native English speakers get confused and use biannual to mean biennial—especially since very few events are biennial, so this word is not used often. However, this is still officially incorrect. It should be easy to remember that biannual refers to the more common idea “twice a year” since the word annual means “once per year” but there is no word *ennial.