A viceroy /ˈvaɪs.rɔɪ/ is a royal official who runs a country, colony, or city province (or state) in the name of and as representative of the monarch. The term derives from the Latin prefix vice-, meaning "in the place of" and the French word roi, meaning king.
In Star Wars canon, the most famous holder of that rule was of course Princess Leia Organa's adoptive father, Bail Organa:
Formally styled as His Serene Highness, Prince Bail Organa, First Chairman and Viceroy of Alderaan - from "Bail Prestor Organa" Wookiepedia article
QUESTION: Given the fact that Bail Organa had, as one of his titles, "Viceroy Bail Organa", who was the King/Sovereign that he ruled in the name/place of?
This is especially puzzling since he was also a Prince Consort, ruling by virtue of being married to Queen Breha Organa of Alderaan. So the Viceroying wasn't on her behalf since Prince Consort is the title conveying that specific relationship.
Please note that other uses of the title Viceroy in SW canon are less weird but also unclear - Viceroy Nute Gunray didn't seem to have a sovereign either, though he was supposedly reporting to "Trade Federation Directorate".