A large operator symbol representing the union of multiple sets or collections in set theory.
Examples
Union of a finite collection of sets
A \cup B \cup C = \bigcup_{i=1}^3 A_i
Union of an infinite sequence of sets
\bigcup_{n=1}^{\infty} (0, \frac{1}{n}) = (0, 1)
Union of indexed family of sets
\bigcup_{\alpha \in I} X_\alpha = \{x : x \in X_\alpha \text{ for some } \alpha \in I\}