Mathematically, e stands for natural constant. Natural constants are also constants in mathematics. The natural constant e is an infinite acyclic decimal with a size of about 2.7 1828 1828459. It is worth mentioning that e is not only a mathematical constant, but also the base of natural logarithmic function. E is sometimes called Euler number, named after Swiss mathematician Euler, and there is a rare name called Napier constant to commemorate the introduction of logarithm by Scottish mathematician John Napier. It is one of the most important constants in mathematics, just like pi and imaginary units I and E.