Pi is the ratio of the circumference of a circle to its diameter.
This ratio holds true for any circle, regardless of its size.
A simple experiment can be done to verify this for yourself.
Grab a piece of paper and, with the help of a compass, draw a circle.
Be sure to mark the center of your circle. Now, grab a string and
wrap it exactly one time around the circle. Trim the string so
that it fits perfectly around the edge of your circle. This string
is measuring the circumference of your circle. Measure the length
of the string. Next, measure the diameter of the circle (this
can be done by measuring the distance from any point on the circle,
through the center of the circle, to a point on the opposite side).
Now divide the circumference of the circle by the diameter. This will
be approximately 3.14, no matter the size of the circle that you drew.
If you could measure perfectly, your answer would be pi exactly (Bogart).
Pi is most commonly approximated to 3.14. However, pi is irrational,
which means that it is infinite when expressed as a decimal and that it never becomes repetitive.
To the first 18 decimal places, pi is 3.141592653589793238. Fun fact:
the number 123456 doesn't appear anywhere in the first million digits
of pi, making pi an extremely unique number (Mometrix).