The Collaborative International Dictionary of English v.0.48:

onside kick \onside kick\ (Football)
   A kickoff in which the kicking team attempts to kick the ball
   only so far as is legally required (beyond the 50-yard line),
   so as to have a better chance to recover the kicked ball
   itself; -- it is a risky type of play used only when the
   kicking team is losing and there is little time left to play.
   [PJC]