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Member of a set
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.
