Why $\operatorname e$ was named e ? What is the history and reason behind it?

Question

Why the constant that Euler discovered has symbol $\operatorname e$ and not other symbols? What is the history and reason behind it?

Thanks.

Answer 1

Why did he [the Swiss mathematician Leonhard Euler $\mathit{(1707–1783)}$] choose the letter $e$? There is no general consensus. According to one view, Euler chose it because it is the first letter of the word exponential. More likely, the choice came to him naturally as the first “unused” letter of the alphabet, since the letters $a$, $b$, $c$, and $d$ frequently appear elsewhere in mathematics. It seems unlikely that Euler chose the letter because it is the initial of his own name, as occasionally has been suggested: he was an extremely modest man . . .

— Eli Maor, $e$: The Story of a Number (Princeton, N.J.: Princeton University Press, $1994$)