Sovereign describes an entity holding supreme authority — answerable to no higher power within its domain — and, as a noun, names the one who holds it: a monarch, a state, or, in modern usage, a self-governing person.
The word reaches English in the late thirteenth century as soverain, from Old French soverain ("highest, supreme, chief"), which descends from Vulgar Latin superanus, built on Latin super, "over." The -g- is an intruder, inserted by folk-etymological association with reign, a word to which it is unrelated; Middle English also tended to add an unetymological -t, as it did to pheasant and tyrant. The Middle English Compendium lists thirty-eight spellings. Elizabeth I, who was one, spelled it seven different ways, and Milton printed it sovran, as though it came from Italian sovrano.
The word carries senses well beyond rule. From the late fourteenth century a sovereign remedy is one potent in the highest degree — the reading that survives in "a sovereign cure." From the late fifteenth century a sovereign is also an English gold coin, revalued at one pound in 1817. And in the political writing of the seventeenth and eighteenth centuries the sovereign is often not the monarch at all but the populace in its collective and legislative capacity; John Adams used it that way in arguing that in America "the right of sovereignty resides indisputably in the body of the people."
Sovereign is the concrete term; 📝sovereignty is the abstract quality it holds. Both now extend to the individual — the capacity to govern one's own choices without subordination to outside influence — which is the sense at work in frameworks such as 📝Relational Sovereignty.