13. A term or boundary is the extremity of any thing.
14. A figure is that which is enclosed by one or more boundaries.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/5/57/The_Elements_of_Euclid_for_the_Use_of_Schools_and_Colleges_-_1872_page_3a.png/200px-The_Elements_of_Euclid_for_the_Use_of_Schools_and_Colleges_-_1872_page_3a.png)
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.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/df/The_Elements_of_Euclid_for_the_Use_of_Schools_and_Colleges_-_1872_page_3b.png/100px-The_Elements_of_Euclid_for_the_Use_of_Schools_and_Colleges_-_1872_page_3b.png)
24. Of three-sided figures,
An equilateral triangle is that which has three equal sides:
1—2