The size of the wheel as measured across its center.
The width of the wheel as measured across its barrel from the front rim to the rear rim.
The bolt pattern indicates the arrangement of lug holes and consists of two elements: the number of bolts, or lugs, and the distance between them across the center of the wheel (in millimeters).
The offset of a wheel is what locates the tire and wheel assembly in relation to the suspension. More specifically, it is the measured distance between the hub mounting surface and the centerline of the rim. It can be either positive or negative, and is typically measured in millimeters.