A biennial timeshare can be used every other year, as opposed to the more popular every year timeshare. In this case, odd or even years are assigned to the ownership.
Biennial timeshare ownerships are simply ownerships that allow usage every other year. At the time of purchase you will be assigned either even or odd years, and this usage year is written into the deed of your ownership. This means you usually won’t be able to change your years of usage once you have purchased the ownership, so be sure to plan ahead and choose the use pattern that will work best for you.
A common misnomer for biennial is biannual, this is technically an incorrect way of describing a timeshare ownership, but just know these terms are considered interchangeable.