The Collaborative International Dictionary of English v.0.48:

Whiplash injury \Whip"lash` in"ju*ry\, n.
   an injury to the neck caused by the sudden motion of the head
   backward and forward, as occurs to the occupants of a vehicle
   hit from behind by another vehicle.
   [PJC]

WordNet (r) 3.0 (2006):

whiplash injury
    n 1: an injury to the neck (the cervical vertebrae) resulting
         from rapid acceleration or deceleration (as in an
         automobile accident) [syn: whiplash, whiplash injury]