Hijack

Were you looking for the gameplay mechanic, or the Halo 2 (Windows Vista) Carjacking achievement?

Hijack (Carjacking in Halo 2 and Hijacker in Halo 3 and Halo 3: ODST firefight) is a medal in Halo 2, Halo 3, Halo 3: ODST, Halo: Reach, Halo 4, and Halo 5: Guardians. It is awarded for hijacking an enemy occupied vehicle and taking it from them. It is available in Campaign (Halo 3: ODST, Halo: Reach, and Halo 5: Guardians), Multiplayer, firefight, and Spartan Ops.

This medal is represented by a U-turn sign on a bronze cross, except in Halo 4 and Halo: The Master Chief Collection where the cross is grey and turned 45 degrees. In Halo 5: Guardians it is represented by the roadsign only.

After you steal an occupied vehicle, a message appears on your screen. In Halo 2, that message reads: "You boarded an enemy vehicle", while in Halo 3 and every game since, it reads: "You hijacked (player's name)".

Trivia

Earning this medal in Halo 2 for Windows Vista multiplayer unlocks the Carjacking achievement.
Prior to the shutdown of Xbox 360 Halo online services on January 13, 2022, earning this medal in a Halo 3 free-for-all playlist by hijacking a vehicle within 10 seconds of being hijacked from that same vehicle unlocked the Maybe Next Time Buddy Achievement.
The medal has a Bungie difficulty rating of 2 out of 10.
If you hijack and commandeer a person on the back of a Warthog who is facing away from you, you will also get an Assassination medal and the person will almost instantly die. However should they be facing you, they will only lose their shields and be stunned.