The number "e" is one of the most important numbers in mathematics.
It is often called Euler's number after Leonhard Euler.
The first few digits are:
2.7182818284590452353602874713527... (and more)
It is the base of the natural logarithm.
It can be calculated many ways, for example the value of (1 + 1/n)n approaches e as n gets bigger and bigger:
See: Logarithm