[1][2][3][4] But for a few authors a singleton is also considered an unordered pair, although today, most would say that {a, a} is a multiset.
It is typical to use the term unordered pair even in the situation where the elements a and b could be equal, as long as this equality has not yet been established.
An unordered pair is a finite set; its cardinality (number of elements) is 2 or (if the two elements are not distinct) 1.
In axiomatic set theory, the existence of unordered pairs is required by an axiom, the axiom of pairing.
More generally, an unordered n-tuple is a set of the form {a1, a2,... an}.