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]

WordNet (r) 3.0 (2006):

decennary
    n 1: a period of 10 years [syn: decade, decennary,
         decennium]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

DECENNARY, Eng. law. A town or tithing, consisting originally of ten
families of freeholders. Ten tithings composed a hundred. 1 Bl. Com. 114.