TITLE: What does the symbol $\in$ mean?
QUESTION [6 upvotes]: I am currently studying discrete mathematics, and I have very little background knowledge, and I was wondering what the symbol "$\in$" means.
The following is where I encountered this symbol:

REPLY [4 votes]: $\in$ means "is an element of", or more simply, "is in", which is why it is \in in LaTeX.
For example:

"Let $x \in \mathbb{Z}$" = Let $x$ be an integer.
"Let $x \in \{0, 1\}$" = Let $x$ be either $0$ or $1$.

Note, that there is also the notation $\ni$, which means the same thing in the other order. For example, $x \in \mathbb{Z}$ is equivalent to $\mathbb{Z} \ni x$.
This is read as "contains", and has LaTeX code \ni.