In this context, "speed of light" really refers to the speed supremum of information transmission or of the movement of ordinary (nonnegative mass) matter, locally, as in a classical vacuum. Thus, a more accurate description would refer to formula_1 rather than the speed of light per se. However, light and other massless particles do theoretically travel at formula_1 under vacuum conditions and experiment has nonfalsified this notion with fairly high precision. Regardless of whether light itself does travel at formula_1, though formula_1 does act as such a supremum, and that is the assumption which matters for Relativity.