# Pauling's equation on the relationship between Electronegativity and Percentage Ionic Character [closed]

In Paulings eqn:

$$\%~\ce{ionic~character} = \left[ 1−e^{−(Δχ/2)^2}\right] \times 100$$

What is $e$?

I for the love of me can't find it anywhere

% Ionic character $= (1 − e^{−(\frac{Δχ}{2})^2})×100$
where the $−(\frac{Δχ}{2})^2$ term is raised as a power of the natural exponent 'e'.