The Collaborative International Dictionary of English v.0.48:
Decennary \De*cen"na*ry\, n.; pl. Decennaries. [L. decennium a period of ten years; decem ten + annus a year.] 1. A period of ten years. [1913 Webster] 2. (O. Eng. Law) A tithing consisting of ten neighboring families. --Burrill. [1913 Webster]