A decennary is rare word for a ten-year period. In other words, it's a decade. Call a decade a decennary if you’re feeling like an old fashioned British lawyer.
Like decade and decagon (polygon with ten sides), decennary comes from the Latin root decem for “ten.” A hundred years is made up of ten decennaries. People tend to make a big deal about birthdays that end a decennary, like turning 10, 20, or 30, etc.. A decennary is also known as a decennium. An old definition for this word is “ten neighboring families,” but that definition is from a few decennaries ago.