Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
semicentennial is a noun and adjective meaning "the 50th anniversary (or the celebration of it)". It carries 2 recorded senses.
the 50th anniversary (or the celebration of it)
of or relating to or marking the 50th anniversary
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for semicentennial come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.