Saltpetre is identified without dispute as potassium nitrate, KNO3, one of the more chemically stable and well behaved of the traditional salts, and one whose modern identity was worked out relatively early once eighteenth century chemists began systematically distinguishing the alkali metals and the nitrate radical. Its oxidizing behavior, releasing oxygen readily when heated with a fuel, is exactly what makes it central to both gunpowder and the strong acid distillations that depend on its nitrogen content; historians of chemistry including John Emsley treat saltpetre as a clean case where the traditional name, the traditional preparation, and the modern formula line up with no real ambiguity left to resolve.
The one historical wrinkle worth noting is that early modern saltpetre, gathered from natural efflorescence rather than manufactured from pure reagents, was often contaminated with related salts, calcium nitrate and sodium nitrate especially, which refiners removed by exploiting potassium nitrate's more favorable crystallization behavior, a purification chemistry the refiners understood empirically long before anyone could explain it in terms of solubility curves. That empirical skill, refining a militarily and economically vital salt to reliable purity with no atomic theory to guide the work, is a useful reminder of how much practical chemistry pre-modern refiners achieved through careful, repeated observation alone.