Why do we call it "pi"?

The use of the Greek letter π for this constant is derived from Greek words like περιφέρεια (for periphery) and περίμετρον (for perimeter) starting with this letter.

The name of the Greek letter π is pi, and this spelling is used in typographical contexts where the Greek letter is not available or where its usage could be problematic. When referring to this constant, the symbol π is always pronounced like "pie" in English, the conventional English pronunciation of the letter.

Because pi is the first letter in the Greek word perimitros, which means "perimeter." It's a special number.