The Collaborative International Dictionary of English v.0.48:

flypast \fly"past\ (fl[imac]"p[a^]st), n.
   1. the upper level of a crossing of two highways at different
      levels; same as flyover; called in the United States an
      overpass. [Brit.]

   Syn: overpass, flyover.
        [WordNet 1.5]

   2. a low-altitude flight (usually of military aircraft) over
      spectators on the ground.

   Syn: flyover.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

flypast
    n 1: a flight at a low altitude (usually of military aircraft)
         over spectators on the ground [syn: flyover, fly-by,
         flypast]