E

Actually, e is not "Euler's Constant", it's "Euler's Number".  Euler's Constant is usually written with a γ (lower case gamma) and is about 0.577 .  The link below explains it and it lists several other numbers named after Euler.  So to avoid confusion, I just took out his name.  I also changed the index.  Napier's Constant is correct, and I used that in a few places, but it's a similarly obscure term.  So I gravitated toward "The base of natural logarithms, e".  People should get that.

http://en.wikipedia.org/wiki/List_of...2.80.94numbers