A Chairman is the presiding official of an assembly, meeting, committee, or board. In a corporation, the Chairman of the board of directors can also hold the position of Chief Executive Officer.
In 2374, Nilva was Chairman of the Slug-o-Cola beverage company. (DS9: "Profit and Lace")
In 2375, Koval was the Chairman of the Romulan Tal Shiar. (DS9: "Inter Arma Enim Silent Leges")