The A.T.V. (short for All Terrain Vehicle) is Lara's custom motorcycle used during the events of Tomb Raider: Underworld.
Lara uses this motorcylce during missions which requires her to cover a lot of ground, namely, Mexico and Jan Mayen island.
Lara later uses the bike to get to a point in the arctic where Natla claimed the entrance to Helheim is. The bike was left there as Lara used a portal to leave Helheim.
The ATV is completely custom, featuring his suspension, dual exhausts, and a massive engin. One of the most distinctive features is the large square head.