The circle with centre O and radius r is the set of all points in the plane whose distance from O is r.

The line segment OA (interval OA) is also called a radius of the circle.

Putting the point of a pair of compasses at the centre and opening the arms to the radius can draw a circle.

Pi is the name of the Greek letter that is used to denote the ratio of the circumference of any circle to its diameter. The number is irrational, but is a rational approximation accurate to 2 decimal places. The decimal expansion of begins

There is a very long history of attempts to estimate accurately. One of the early successes was due to Archimedes (287–212 BC) who showed that

The decimal expansion of has now been calculated to at least the first places.