13. A term or boundary is the extremity of any thing.
14. A figure is that which is enclosed by one or more boundaries.
15. A circle is a plane figure contained by one line, which is called the circumference, and is such, that all straight lines drawn from a certain point within the figure to the circumference are equal to one another:
16. And this point is called the centre of the circle.
17. A diameter of a circle is a straight line drawn through the centre, and terminated both ways by the circumference.
[A radius of a circle is a straight line drawn from the centre to the circumference.]
18. A semicircle is the figure contained by a diameter and the part of the circumference cut off by the diameter.
19. A segment of a circle is the figure contained by a straight line and the circumference which it cuts off.
20. Rectilineal figures are those which are contained by straight lines:
21. Trilateral figures, or triangles, by three straight lines:
22. Quadrilateral figures by four straight lines:
23. Multilateral figures, or polygons, by more than four straight lines.
24. Of three-sided figures,
An equilateral triangle is that which has three equal sides: