Tercentennial is a word that describes a 300-year anniversary or celebration. While not a commonly used term, there are several synonyms that can be used instead. One such word is tricentennial, which has the same meaning but is more commonly used. Another option is the phrase "300-year anniversary," which is straightforward and easy to understand. Alternatively, "300th-year celebration" or "300th-anniversary commemoration" can also be used to convey the same idea. Regardless of which synonym is used, the important thing is to acknowledge and celebrate the historical significance of a milestone that marks three centuries of existence.