ter·cen·ten·a·ry 
(tûr′sĕn-tĕnə-rē, tər-sĕntə-nĕr′ē)
(tûr′sĕn-tĕnə-rē, tər-sĕntə-nĕr′ē)n. pl. ter·cen·ten·a·ries
A 300th anniversary or its celebration.
adj.
Of or relating to a span of 300 years or to a 300th anniversary.
[Latin ter, thrice; see TERN2 + CENTENARY.]