The Collaborative International Dictionary of English v.0.48:

Centenary \Cen"te*na*ry\, a. [L. centenarius, fr. centum a
   hundred.]
   1. Relating to, or consisting of, a hundred.
      [1913 Webster]

   2. Occurring once in every hundred years; centennial.
      "Centenary solemnities." --Fuller.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Centenary \Cen"te*na*ry\, n.; pl. Centenaries.
   1. The aggregate of a hundred single things; specifically, a
      century. "Every centenary of years." --Hakewill.
      [1913 Webster]

   2. A commemoration or celebration of an event which occurred
      a hundred years before.
      [1913 Webster]

WordNet (r) 3.0 (2006):

centenary
    adj 1: of or relating to or completing a period of 100 years;
           "centennial celebration" [syn: centennial, centenary]
    n 1: the 100th anniversary (or the celebration of it) [syn:
         centennial, centenary]