de·cen·ni·um 
(dĭ-sĕnē-əm)
(dĭ-sĕnē-əm)n. pl. de·cen·ni·ums or de·cen·ni·a (-sĕnē-ə)
A period of ten years; a decade.
[Latin, from decennis, lasting for ten years : decem, ten; see dek in the Appendix of Indo-European roots + annus, year; see at- in the Appendix of Indo-European roots.]