Lecture
This lecture covers the definition of the union of sets, where the union of sets X and Y includes elements that belong to X or Y. It explains how the union is not an exclusive 'or'. The properties of union are compared to addition, showing that it is associative and commutative. The lecture also discusses operations on sets, such as the union of multiple sets and the complement of a set. The proof of the properties is demonstrated through double inclusion. The intersection of sets X and Y is defined as the subset of elements present in both X and Y.