Il numero di Eulero (115)

The exponential function is constructed by raising to the power of a particular number, e, called Euler's number. The value of this number is: e = 2.718281828459045235360287471352662... The question that naturally arises is: why exactly this value? This number, among other things, is irrational, and what's more, transcendental; therefore, it's not even possible to write a polynomial equation to which it would be a solution. But then, how do we define it?