A Chairman is the presiding official of an assembly, meeting, committee or party. In a corporation, the Chairman of the board of directors can also hold the position of Chief Executive Officer.
In China, Mao Zedong was Chairman of the Communist Party from 1935 to 1976 - and Chairman of its state, the People's Republic, from 1949 to 1959. (ENT: "Storm Front, Part II")
On Ekos, Eneg was the Chairman of the Nazi Party during the 2268 crisis in state leadership. (TOS: "Patterns of Force")
Pascal Fullerton was the chairman of the New Essentialists in the 2370s. (DS9: "Let He Who Is Without Sin...")
Quark and Rom were both chairmen of the short-lived Ferengi Benevolent Association in 2371. (DS9: "Prophet Motive")
In 2374, Nilva was Chairman of the Slug-o-Cola beverage company. (DS9: "Profit and Lace")
In 2406 Vaebn was the chairman of the Romulan Tal Shiar and was instrumental in preventing a coup in the Romulan Star Empire when he assisted Praetor Vrih in holding his position against a plot put forward by Senator Terrah