A president is a leader of an organization, company, club, trade union, university, country, a division or part of any of these, or, more generally, anything else.
Etymologically, a president is one who presides, (from Latin prae- "before" + sedere "to sit"; giving the term praeses). Originally, the term referred to the presiding officer of a ceremony or meeting (i.e., chairman), but today it most commonly refers to an official. Among other things, "President" today is a common title for the heads of state of most republics, whether popularly elected, chosen by the legislature or by a special electoral college.