What does $\in$ mean?

Question

I'm reading a textbook on complex analysis and I've come across notation using this ($\in$) symbol. In the context of "an argument of $z = x + iy$ is a number $\phi \in \mathbb R$ such that $x = r\cos\phi$ and $y = r\sin\phi$" what does this symbol mean?

Answer 1

$\phi\in R$ just means that $\phi$ is an element of $R$.

In this case it would mean that $\phi$ is an element of the set of real numbers, $\mathbb{R}$.

Answer 2

As $\in$ has $\LaTeX$ code of \in, $\phi\in\mathbb{R}$ means $\phi$ is element of set of real number $\mathbb{R}$.

Answer 3

$\in$ means "is an element of", or "is in". If I had a set $S = \{1,2,3\}$, then $1 \in S$, but $4 \notin S$.