Element

• Definition: An element is a single object that belongs to a set.
• Notation: $a\in A$ means ”$a$ is an element of the set $A$.”
• Example:
  • $3\in \{1,2,3,4\}$
  • $\mathbb{N}\subseteq \mathbb{Z}$, but 5 is an element, not a set: $5\in \mathbb{N}$.