tercentenary

UK: /ˌtɜːsɛnˈtiːnərɪ/

US: (tûr′sen ten′ə rē, tûr sen′tn er′ē; esp. Brit. tûr′sen tē′nə rē)

Inflections of 'tercentenary' (n): npl: tercentenaries

WordReference Random House Unabridged Dictionary of American English © 2025

ter•cen•ten•ar•y  (tûr′sen ten′ə rē, tûr sen′tn er′ē; esp. Brit. tûr′sen tē′nə rē), adj., n., pl. -nar•ies. 

1. tricentennial.

Also, ter′cen•ten′ni•al. 

• ter- + centenary 1835–45

Collins Concise English Dictionary © HarperCollins Publishers::

tercentenary / ˌtɜːsɛnˈtiːnərɪ/, tercentennial / ˌtɜːsɛnˈtɛnɪəl/ adjective

1. of or relating to a period of 300 years
2. of or relating to a 300th anniversary or its celebration

noun ( -tenaries, -tennials)

1. an anniversary of 300 years or its celebration