1. The Areas of Circles are to each other as the squares of their diameters; any circle twice the diameter of another contains four times the area of the other.

2. The Radius of a circle is a straight line drawn from the centre to the circumference.

3. The Diameter of a circle is a straight line drawn through the centre, and terminated both ways at the circumference.

4. A Chord is a straight line joining any two points of the circumference.

5. An Arc is any part of the circumference.

6. A Semicircle is half the circumference cut off by a diameter.

7. A Segment is any portion of a circle cut off by a chord.

8. A Sector is a part of a circle cut off by two radii.