Set Theory Definitions Set – is a collection of objects or elements. Elements – the objects that make up a set. Null Set (empty set) – a set with no elements (it is a subset of every set). Union- The set of all the elements in two or more given sets. Intersection – The set of all the elements that are common to two or more given sets. Disjoint sets – A set with no elements in common. Universal Set (Parent Set) – A set containing all elements considered in a given problem; serves as an encompassing set for various subsets. Subset – A set whose elements are all members of another set. Complement – The set of all elements in the universal set that are not in a given set, which is a subset of the universal set.