Lug spacing is the internal distance, measured in millimeters, between the two pins on which the strap or bracelet attaches to the case. It should not be confused with the external width of the lugs themselves: that is an aesthetic measurement, this is a functional one.
At the workbench I measure it with a caliper placed inside the lugs, flush with the lower edge of the case, where the strap makes contact with the metal. Half a millimeter error here creates play, lateral tension on the links, or worse, a strap that won't fit at all. I've seen customers ruin new straps by taking the external measurement instead of the internal one.
The most common sizes today are 18, 19, 20, 22, and 24 mm. The 20 mm covers most sports watches and medium-sized tool watches. The 22 mm is frequent on more robust divers. The 24 mm is historically linked to certain Swiss manufacturers that produce large-diameter cases. Odd sizes like 19, 21, or 21.5 mm do exist and complicate matters: the aftermarket for these widths is more limited, prices rise, and choice narrows.
A detail that technical sheets often ignore: lug spacing can vary slightly between the upper and lower lugs if the case is asymmetrical or if the manufacturer has designed a shaped strap. In that case the relevant measurement is the one at the narrowest point, because that's where the strap must pass without forcing.
The English term lug spacing is the most used in forums and international catalogs. Lug width circulates as a synonym but technically describes the width of the lug alone, not the space between the two: the distinction is subtle but real. In Italian you also find passo delle anse and misura anse, less precise but understandable.