ter ‧ cen ‧ te ‧ na ‧ ry /ˌtɜːsenˈtiːnəri $ ˌtɜːrsenˈtenəri, tɜːrˈsent ə neri/ BrE AmE noun ( plural tercentenaries ) [countable] British English
[ Date: 1800-1900 ; Language: Latin ; Origin: ter 'three times' + English centenary ]
the day or year exactly 300 years after a particular event