Vocabulary - Element

An element is an item that belongs to a set. If $A = \{1, 2, 3\}$, then $2$ is an element of $A$.

You write $2 \in A$ to say "$2$ is in $A$." You write $5 \notin A$ to say "$5$ is not in $A$."

The symbol $\in$ is read as "is an element of" or just "in." This symbol appears everywhere in set theory.