e is Approximately 2.7

Definition: e is a mathematical constant that is approximately equal to 2.7. It is an irrational number, but if we truncate it to 20 decimals it would be 2.7182818284590452353. The function e^x has the same value as the slope of the tangent line, for all values of x. It is used in several PHP functions, such as exp (x), a function which calculates e to the power of x.