SEMICENTENNIAL
semicentennial, semicentenary
(adjective) of or relating to or marking the 50th anniversary
semicentennial, semicentenary
(noun) the 50th anniversary (or the celebration of it)
Source: WordNet® 3.1
Etymology
Adjective
semicentennial (not comparable)
Occurring every 50 years
Noun
semicentennial (plural semicentennials)
A 50th anniversary.
Source: Wiktionary
Sem`i*cen*ten"ni*al, a.
Definition: Of or pertaining to half of a century, or a period of fifty
years; as, a semicentennial commemoration.
Sem`i*cen*ten"ni*al, n.
Definition: A fiftieth anniversary.
Source: Webster’s Unabridged Dictionary 1913 Edition