The distance between the wheel mounting surface and the wheel center line. Determines how wheels sit relative to the fender and chassis.
Wheel offset is the distance between the wheel mounting surface (where it bolts to the hub) and the wheel center line (the geometric center of the wheel width). Offset is measured in millimeters and can be positive (mounting surface toward the outside of the wheel), zero (mounting surface at the wheel center), or negative (mounting surface toward the inside of the wheel).
Most modern passenger cars have positive offset wheels (mounting surface toward the outside, wheel pulled inward against the suspension). The result is the wheel face being inboard relative to the fender. Lower offsets push the wheel face outward, achieving the flush or aggressive fitment popular in stance and tuner culture. Negative offset wheels (typical of older trucks and racing cars) have the wheel face well outboard of the fender.
Changing wheel offset has practical consequences. Lower offset increases scrub radius (the distance between the steering axis and the tire contact patch), which affects steering feel. It can also cause clearance issues with brakes, suspension components, and fenders. The visual benefit of lower offsets (stance, hellaflush) is balanced against handling and clearance trade-offs. Quality wheel manufacturers offer multiple offset choices for popular platforms.