A tercentenary is a 300th anniversary — the year or the day exactly three hundred years after something happened, and usually also the celebration held to mark it: a university's tercentenary, the tercentenary of a composer's birth, a tercentenary concert. The word works as a noun ("they are planning the tercentenary") and as an adjective before another noun ("a tercentenary exhibition"). It is formal and written: you meet it on a plaque, in a museum leaflet, in a newspaper report about a cathedral, not in conversation. In ordinary speech people simply say "the three hundredth anniversary", and that is never wrong. **Tercentennial** means the same thing and is the usual form in American English; British English leans to tercentenary. It belongs to a small family you can guess your way through once you see the pattern: centenary is 100, bicentenary 200, tercentenary 300, quatercentenary 400. Say it ter-sen-TEE-na-ree in British English, though you will hear ter-SEN-ten-ree too.

三百週年（紀念）。中文平常就講「三百週年」；tercentenary 屬於書面、正式場合的字。