XVII.
A diameter of a circle is a straight line drawn through the centre, terminated both ways in the circumference.
XVIII.
A semicircle is the figure contained by the diameter, and the part of the circle cut off by the diameter.
XIX.
A segment of a circle is a figure contained by a straight line, and the part of the circumference which it cuts off.
XX.
A figure contained by straight lines only, is called a rectilinear figure.
XXI.
A triangle is a rectilinear figure included by three sides.
XXII.
A quadrilateral figure is one which is bounded by four sides. The straight lines 
and 
connecting the vertices of the opposite angles of a quadrilateral figure, are called its diagonals.
XXIII.
A polygon is a rectilinear figure bounded by more than four sides.
