The symbol used by mathematicians to represent the ratio of a circle's circumference to its diameter is the lowercase Greek letter π, sometimes spelled out as pi, and derived from the first letter of the Greek word perimetros, meaning circumference.

Pi (π) Definition: Pi is a number - approximately 3.142. It is the circumference of any circle divided by its diameter. The number Pi, denoted by the Greek letter π - pronounced 'pie', is one of the most common constants in all of mathematics. It is the circumference of any circle, divided by its diameter.

piz. za. A baked pie of Italian origin consisting of a shallow breadlike crust covered with toppings such as seasoned tomato sauce, cheese, or sausage. [Italian, pie, tart, pizza, of Germanic origin; akin to Old High German bizzo, pizzo, bite, morsel; see bheid- in Indo-European roots.]