Wiktionary
n. A group of people chosen by the shareholders of a company to promote their interests through the governance of the company and to hire and supervise the executive directors and CEO.
Wikipedia
A supervisory board or supervisory committee, often called board of directors, is a group of individuals chosen by the stockholders of a company to promote their interests through the governance of the company and to hire and supervise the executive directors and CEO.
Corporate governance varies between countries, especially regarding the board system. There are countries that have a one-tier board system (like the U.S.) and there are others that have a two-tier board system like Germany.
In a one-tier board, all the directors (both executive directors as well as non-executive directors) form one board, called the board of directors.
In a two-tier board there is an executive board (all executive directors) and a separate supervisory board (all non-executive directors).