The Collaborative International Dictionary of English v.0.48:

Semicentennial \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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Semicentennial \Sem`i*cen*ten"ni*al\, n.
   A fiftieth anniversary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

semicentennial
    adj 1: of or relating to or marking the 50th anniversary [syn:
           semicentennial, semicentenary]
    n 1: the 50th anniversary (or the celebration of it) [syn:
         semicentennial, semicentenary]