PI is a variable.

http://answers.yahoo.com/question/index?qid=20080708162347AAh4RJ4
A variable is an unknown

but no one ever calculated or seen value of Pi, so Pi must be unknown. Besides, even on different SBCL implementations Pi has different values.

>>14
It can be expressed.  I propose a number system with base equal to circumference divided by the diameter radius of any circle.  In that system, $\tau$ will be written as $10$, and $\pi$ will simply mean $\frac{\tau}{2}$.