π (a symbol spelled pi and pronounced like pie) is a number representing the ratio of a circle's circumference to it's diameter.
It can also informally refer to a girl with curly brown hair who appears at the start of I Am Alexander Graham Bell. She is wearing a -light green barette, dark-green shorts, and most notably a yellow t-shirt which has the symbol for pi (π) on the front of it.
She recurs at the end of I Am Cesar Chavez as part of the crowd of cheering kids arriving to play at the KidZone