IPA: /tɜːrˈsɛn.tə.ni.əl/
KK: /tɜrˈsɛnˌtɛn i əl/
Relating to a period of three hundred years or a celebration of such an anniversary.
The city held a tercentennial celebration to mark its 300th anniversary.
A celebration or event marking the 300th anniversary of something.
The town held a tercentennial celebration to honor its founding in 1723.
Tercentennial is formed from "ter-" (meaning three), "cent-" (from Latin "centum", meaning hundred), and "-ennial" (meaning relating to years). The word refers to a celebration or event that occurs every three hundred years.
Think of 'three' ('ter-') and 'hundred' ('cent-') to remember that 'tercentennial' refers to something that happens every three hundred years.