Lowercase pi represents the Greek letter $\mathrm{\pi }$, and in most cases it has no special meaning. There are some instances where pi does have special meaning. For instance, the number-theoretic function pi(n). For more information, see NumberTheory[pi].

Uppercase Pi represents the mathematical constant, that is the ratio between the circumference and diameter of a circle, approximately equal to 3.141592654. For information on this math constant, see Pi.

For the product operator, see product.

sin(Pi/2);

$1$

evalf[20](Pi);

$3.1415926535897932385$

NumberTheory[pi](1000);

$168$

product( 4*n^2 / (4*n^2-1), n=1..infinity );

$\frac{\mathrm{\pi }}{2}$