Pi, a Transcendental Number
Perhaps the most familiar of all the irrational numbers is the number π, which measures the ratio of the circumference of a circle to its diameter. Since it is irrational (although this fact has not always been known) it cannot be expressed by using a finite number of digits either as a fraction or a decimal. We therefore denote it by a special symbol for convenience. This symbol π, pronounced ‘pi’, was not actually used until the eighteenth century (in fact it was first written by Euler) although interest in the quantity which it represents, and in the question concerning its possible irrationality, goes back much further.