Pi = 3.14159265359, or usually shorted to something like 3.14
Actually, pi is proven to just keep going forever with decimal digits. This pattern is also unrepetitive, or having no patern to it.
Or as Wikipedia says it, "It's [pi's] decimal representation never ends and never settles into a permanent repeating pattern. The digits appear to be randomly distributed although no proof of this has yet been discovered."
Pi is a ratio of a circle's circumference to it's diameter, as shown below.![]()
This means that a circle's circumference is a little over 3 times as big as it's diameter.
This also means that you can write pi as pi = circumference/diameter:

Because of this, you will often use pi when your dealing with circles.
Wikipedia quote and picture taken from: http://en.wikipedia.org/wiki/Pi