Pauling's equation on the relationship between Electronegativity and Percentage Ionic Character
'e' is a natural exponential function, so not an actual measurement.

The equation is correctly written as:

% 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'.

Source: The University of the West Indies page Lecture 3. Polarizability, you'll need to scroll down the page a bit to the section labelled 'Bond character based on electronegativity differences'.