Consider any circle of diameter (D) and circumfenence (C)
Then pi = C/D It can be proved that pi is a constant for any circle.
pi approx 3.14159265 (to 8 decimal places)
pi is irrational (i.e. cannot be expressed as a fraction: p/q :p,q in ZZ)
pi is also transcendental (i.e. is not the root of any polynomial equation with rational coefficients)
pi is one of the most common real constants in all of mathematics.