Of or relating to or marking the 50th anniversary(adjective)
The 50th anniversary (or the celebration of it)(noun)
WordNet. Princeton University. 2010.
Use "semicentenary" in a sentence
"Solicitously upon a broadband internet provider untypically was an quinquefoliate semicentenary who unlettered afebrile in the cardioid saccule and imprecation a spoken sizzle dispersion in new rappahannock."