The number π is a mathematical constant, the ratio of a circle's circumference to its diameter, commonly approximated as 3.14159 (3.14).It has been represented by Greek letter "π" since the mid - 18 th century, through it is also sometimes spelled out as "pi".
Ancient Egyptians and Greeks know the ratio of the circumference to the diameter of a circle is the same for all circles and is slightly higher than 3. Though the estimate of ancient civilizations was close to pi, it was not until Archimedes, a Greek mathematician and physicist, who more accurately calculated pi.