Proper term - Offset or Backspacing?

[clancj]

All depends on the information given. If the desired back-spacing is known, wheel width can be ordered with the specific back-spacing built into it to place the wheel in the desired location.

offset is the difference between the centerline of the wheel (over-all width) and the location of the inner edge of the mounting flange. if the desired wheel width is known, offset can be order into the wheel to place the wheel in the desired location.

"0" offset is absoulte center of the wheel (lip to lip).
"+" offset is the difference between the center-line and mounting flange (toward the inner edge of the wheel).
"-" offset is the difference between the center-line and mounting flange (toward the outer edge of the wheel).
1 inch = 25.4mm

offset: backspacing will vary in relation to wheel width
backspacing: backspacing is fixed regardless of wheel width.

In our world back spacing is king.