Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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