e

is: a numerical constant, whose value to eight decimal places is [M1.5]

can be defined: by

[M1.5]

equivalently can be defined: by

[M1.5]

is the basis: of the exponential function [M1.5]

is used: as the base of natural logarithms. [M1.5]

is: an irrational number. [M1.5]

Contrast with the (italic) used to represent the charge on the proton.