A relational symbol denoting that one set is contained entirely within another set in set theory.
Examples
Basic set inclusion showing natural numbers as a subset of integers
\mathbb{N} \subset \mathbb{Z}
Multiple nested set relationships
A \subset B \subset C \implies A \subset C
Finite set example with explicit elements
\{1, 2\} \subset \{1, 2, 3, 4\}