What is the name of the $\in$ symbol and where does it come from?
Solution 1:
This is the membership relation, but in set theory this is also known as the epsilon relation, and historically the notation was indeed $\varepsilon$.
(For example, I have the book from 1948 by Tarski and Jonsson Cardinal Algebras where such notation is employed.)
According to this page it was Peano who used epsilon. I suppose somewhere around the 1960's or so, when typography was easier to modify the symbol was taking the modern shape of $\in$ (Bourbaki in their set theory book, ca. 1970, were using $\in$).