Locate the Pi key on your calculator. It is designated by Pi, π or sometimes 3.14. This key represents an approximation to an irrational number so useful to mathematics that it is given a name and symbol of its own: π. The symbol π is a Greek letter that is called Pi.
Press the Pi button on your calculator. Depending on the type of calculator you have, different things might happen, but the meaning is the same. On basic calculators striking the Pi key inserts the decimal value of Pi into the display truncated to around 8 figures (3.1415926). Higher end calculators that offer symbolic mathematical inputs may show the symbol π as part of the calculation entered. It is more accurate to use the symbolic form, π, since it is an irrational number (infinitely long and nonrepeating), and truncating it results in some degree of error.
Pi is useful because it relates two common ways of mathematically describing space. Look at the room you are sitting in; if you picked one corner and called it the origin you could accurately describe the location of all the objects in the room by giving the distance from two perpendicular walls and the floor. Cartesian geometry describes physical space in this way, with everything being specified by three numbers (coordinates x, y and z) that represent some distance away from perpendicular planes. But many things aren't easily described by straight lines and planes. Our universe is full of curves, circles and spheres. Sometimes it is much easier to describe a system with spherical coordinates r, θ and φ where r is the distance an object is from the origin and θ (Theta) and φ (Phi) are the two angles (like latitude and longitude) that point r in the right direction. The number Pi relates these, and other curved things, to straight lines.
Consider the relationship between the straight line diameter of a circle and the curve of its circumference. The circumference of a circle is Pi times the diameter. That is, if a circle had a measured diameter (widest measurement possible from edge to edge) of 1 inch, then the distance measured around the outside of the circle would be 3.1415... or Pi. This is how Pi may easily be approximated, by carefully creating a large circle and measuring its circumference and diameter, then dividing the two. Archimedes approximated it using many-sided polygons.
Use Pi to calculate circumference [C = 2 x π x r] (r is radius) and area [A = π x r x r] of circles and the volume of spheres [V = 4/3 x π x r x r x r] and cylinders [V = π x r x r x h] (h is height). Remember that Pi is an irrational number used to connect curved lines to straight ones. It is so pervasive in nature, similar to Phi the golden ratio, that it has acquired an ethereal respect. The use of Pi goes far beyond simple geometric formulas, extending into quantum mechanics and chemistry to astronomy and pure theoretical mathematics.