In a vehicle it's chassis ground, or common return. There is no earth common in a vehicle. You will have people thinking of house wiring. Every component on a vehicle must have a good chassis ground to make the connection back to the battery, which makes the chassis a common return. In a house with earth ground, you can technically run a ground stake into the earth and have a circuit work.

2001 Sportage, 4" RSG lift, Cragers W/31 10.5 Micky Claws, Warn Manual Hubs, 16 gallon Pro Street Fuel Cell, Custom Snorkel, K&N Filter.
1989 Dodge W250.