TITLE: What does "∈" mean?
QUESTION [57 upvotes]: I have started seeing the "∈" symbol in math. What exactly does it mean? 
I have tried googling it but google takes the symbol out of the search.

REPLY [5 votes]: $\in$ means "Element of". 
For example, $a \in A$ means Element of: $a$ is in $A$.
A numeric example would be: $\color{red}3 \in \{1, 2, \color{red}3, 4, 5\}$.