The Skyjacker medal is a medal awarded and achieved in Halo 3, Halo: Reach and Halo 4 multiplayer for hijacking an aircraft (Banshee, Hornet or Falcon). It is represented by a bronze cross with an embedded gold shield, which has an eagle in the center. As this medal is attained, a message appears on the screen, saying "You skyjacked <player>".
A good tactic to get this medal is to throw a Power Drain, or to shoot an overcharged Plasma Pistol bolt to an incoming aircraft. The aircraft will be stunned by your attack, which leaves the player an opportunity to hijack the aircraft.
This is the counterpart of the Carjacker Medal, in which you board a land-based vehicle.
Another way to disable vehicles is to use the EMP function on the grenade launcher. However, the vehicle may be too high for the grenade to reach.