The Collaborative International Dictionary of English v.0.48:
Decennial \De*cen"ni*al\, a. [See Decennary.] Consisting of ten years; happening every ten years; as, a decennial period; decennial games. --Hallam. [1913 Webster]The Collaborative International Dictionary of English v.0.48:
Decennial \De*cen"ni*al\, n. A tenth year or tenth anniversary. [1913 Webster]