Chief executive officer (CEO) is the title generally given to the highest member of day-to-day management in a corporation who usually has the ultimate executive authority within an organization or company.
The CEO usually reports to, and is a member of, the company's board of directors. The CEO may also be the chairman of the board in small companies, but the two roles are often separated in larger organizations, to prevent the company from becoming dominated by a single personality.
In the United States this is a title used by the highest authority within most businesses, regardless of their actual size. In other English speaking countries (most notably British Commonwealth countries) the term is used mainly in publicly traded corporations, and in privately held companies the term Managing Director is much more common.