Draw a circle. Now draw a line from one side of the circle to the opposite side, going through the exact center of the circle. The distance around the circle is the circumfrence. The line through the center is the diameter. If you divide the circumfrence by the diameter, the answer will always be pi.
in math its called pi with no e. its the only decimal number that goes on forever without repeating like a circle has an infinite amount of points. the first three digits are 3.14 and it continues from there.