A relational symbol denoting that one set is a subset of another, including the possibility of equality.
Examples
Basic set inclusion
\{1, 2\} \subseteq \{1, 2, 3\}
Number set relationships
\mathbb{N} \subseteq \mathbb{Z} \subseteq \mathbb{Q} \subseteq \mathbb{R}
Set theory with intervals
[0,1] \subseteq [-1,1] \subseteq \mathbb{R}