The number ¿ is a mathematical constant that is the ratio of the circumference of a circle to its diameter. Because its definition refers to the circle, ¿ is found in many formulas in trigonometry and geometry, especially those related to circles, ellipses, and spheres. It is also found in formulas from other areas of science, such as cosmology, fractals, thermodynamics, mechanics, and electromagnetism. Here we present this number, printed in its first two million digits, in a single volume.