The Collaborative International Dictionary of English v.0.48:

Tercentenary \Ter*cen"te*na*ry\, a. [L. ter thirce + E.
   centenary.]
   Including, or relating to, an interval of three hundred
   years. -- n. The three hundredth anniversary of any event;
   also, a celebration of such an anniversary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tercentenary
    n 1: the 300th anniversary (or the celebration of it) [syn:
         tercentennial, tercentenary, triennial]