The width between the flanges of the wheel (the part the bead of the tyre touches).
The linear distance between the outside of the sidewalls of an inflated tyre without any load on it (this does not include any sidewall decorations).
This is also known as the Aspect Ratio. This is figured on a percentage of the section width. A tyre size of 185/55.15 would have a section width of 185 millimeters and a section height of 55 percent of 185 millimeters.