semi·cen·ten·ni·al /-sɛnˈtɛniəl/
Sem·i·cen·ten·ni·al a. 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. A fiftieth anniversary.
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semicentennial
adj : of or relating to or marking the 50th anniversary [syn: semicentenary]
n : the 50th anniversary (or the celebration of it) [syn: semicentenary]