The definition of a biennium is a period of two years.
Your two-year cell phone contract is an example of a biennium.
nounpl. -·ni·ums or -·nia
a period of two years
Origin of bienniumL, see biennial
nounpl. bi·en·ni·ums, or bi·en·ni·a
A two-year period.
Origin of bienniumLatin bi- two ; see bi- 1. annus year ; see at- in Indo-European roots.