3.14159...

3.14159... (π or pi) is the ratio of a circle's circumference to its diameter.

π is the area of a unit circle. It is also the ratio of the circumference of any convex curve of constant diameter to its diameter (Barbier's theorem).

π is an irrational number and a transcendental number.

22/7 is the best approximation to π, using the ratio of two numbers less than 100.

In 1949, the ENIAC calculated π to 2037 places in 70 hours, without making any mistakes. The current record is nearly 2.7 trillion decimal digits, confirmed in January 2010.

Source: Wikipedia