A commissioner was a member of a commission, often a senior government or police official.
In 1968, a falsified identification card of Gary Seven was signed by NYPD commissioner John Ryan. (TOS: "Assignment: Earth")
High Commissioner Ferris and Assistant Federation Commissioner Nancy Hedford were two noted Federation commissioners, who were addressed as "Commissioner". (TOS: "The Galileo Seven", "Metamorphosis")
Commissioner Bele served as the Chief Officer of Cheron's Commission on Political Traitors. (TOS: "Let That Be Your Last Battlefield")
The Ferengi Commerce Authority consisted of 432 commissioners by 2374. One of those commissioners was Nilva. (DS9: "Profit and Lace")
In 3189, Vos was a commissioner on Trill, answering a hail from USS Discovery on behalf of the Trillian government. (DIS: "Forget Me Not")