1. A circle may be regarded as a collection of points in a plane at a fixed distance from a fixed point. The fixed point is called the centre of the circle. The fixed distance between the centre of the circle and the circumference, is called radius.
2. The perimeter of the circle is referred to as the circumference of the circle.
3. A chord of a circle is a line segment joining any two points on the circumference.
4. An arc of a circle is a part of the circumference.
5. A diameter of a circle is a chord which passes through the centre of the circle.
6. A line, which intersects the circle in two distinct points, is called a secant.
7. A line which has only one point common to the circle is called a tangent to the circle.
8. There is one and only one tangent at a point of the circle.
9. The tangent at any point of a circle is perpendicular to the radius through the point of contact.
10. No tangent can be drawn from a point inside the circle.
11. The lengths of tangents drawn from an external point to a circle are equal.
12. The perpendicular at the point of contact to the tangent to a circle passes through the centre of the circle.
13. Tangents drawn at the end points of a diameter of a circle are parallel.