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semi·cen·ten·ni·al | \ ˌse-mē-sen-ˈte-nē-əl 
, ˌse-ˌmī-, -mi- \
, ˌse-ˌmī-, -mi- \Definition of semicentennial
: a 50th anniversary or its celebration
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Time Traveler for semicentennial
The first known use of semicentennial was in 1859
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