The mathematical constant π (Greek pi) is commonly used in mathematics. It is also known as Archimedes' constant. Pi is an irrational number. Furthermore, it is a transcendental number. Pi is defined as the ratio of the circumference of any circle to its diameter. As all circles are similar and therefore proportional in dimensions, pi is therefore always the same for all circles and is a constant. Consequently, pi can also be viewed as the area of a circle whose radius is one.