At a university, a chair is the position or job of professor.
大学教授 • He has been appointed to the chair of sociology. 他受命任职社会学教授。
If you chair a meeting or a committee, you are the person in charge of it.
担任 (会议、委员会) 主席 He was about to chair a meeting in Venice of E.U. foreign ministers. 他将担任威尼斯欧盟外交部长会议的主席 。