1) Like some censuses 2) Occurring every ten years 3) Of or for ten years 1) Ten year 
• (n.) A tenth year or tenth anniversary. • (a.) Consisting of ten years; happening every ten years; as, a decennial period; decennial games.
De·cen'ni·al adjective [ See Decennary .] Consisting of ten years; happening every ten years; as, a decennial period; decennial games. Hallam. 
of or for ten years. · occurring every ten years.