This article is from the VW Performance FAQ, by with numerous contributions by Jan Vandenbrande others.
A:The distance between the rim's center line and its
mounting surface. From the picture below it should be
obvious why it's important to retain proper offset when
you change rims: tire not centered properly affecting
drivability (negative roll radius changes), bearing load,
rubbing on the struts or wheel arches, etc.
Cross sectional view of a rim:
Center Plane
|\____+____/|
|_____.____ | Street Side of Rim
. //
. //
. || Mounting Face
. ||
>--< Rim Offset
 
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