DIAGONAL

(Gr. iiet, through, and yMviet, angle.) A straight line drawn through a figure, joining two opposite angles. The term is chiefly used In geometry, In speaking of four-sided figures; but it Is also properly applied with reference to all polygons of which the number of sides Is not le than four. Euclid uses the term diameter In the same sense; but modem geometers use diameter only when speaking of curve lines, and diagonal when speaking of angular figures. [s. 347]