TITLE: Why is $e$ the Identity?
QUESTION [13 upvotes]: Some authors use $e$ to be the identity element of a group instead of $1$.  What is the origin of this notation?  Was this before or after we used $e$ to represent the base of the natural logarithm?  If after, is there some connection?

REPLY [10 votes]: We germans (or Kraut, as you might call us) call the identity element "Einheitselement", and hence denote it by $e$ (even though of course not every one does). Similarily it is common to denote a field by $k$ for "Körper"; anyway a great deal of mathematical terminology is of german origin. Likewise the french often denote vector spaces by $E$ ("espace").