Bar Diameter: 25.4 (1") or 31.8 (1.25")
When talking about bars, this number is the Bar Diameter. When talking about stems, it is often called "Clamp Diameter". Clamp diameter can be a bit confusing because a stem has clamps at both ends. However, if only one "clamp diameter" is given, they mean handlebar diameter.
See the chapter in the master guide Handlebars.