Wheel Offset is the distance between the hub mounting face at the back of the wheel and the wheel's centerline.

 

Positive Offset is when the wheel mounting surface is outside of the wheel centerline

 

Negative Offset is when the wheel mounting surface is closer to the inside of the wheel.

 

Offset is measured in millimeters of ET: ET is short for the German word Einpresstiefe which translates to insertion depth. A wheel's offset, or ET is normally stamped or engraved into the wheel.