Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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