Formally, two sets A and B are disjoint if their intersection is the empty set, i.e. if

<math>A\cap B = \varnothing.\,</math>

This definition extends to any collection of sets. A collection of sets is pairwise disjoint or mutually disjoint if any two distinct sets in the collection are disjoint.

Formally, let I be an index set, and for each i in I, let A_i be a set. Then the family of sets {A_i : i ∈ I} is pairwise disjoint if for any i and j in I with i ≠ j,

<math>A_i \cap A_j = \varnothing.\,</math>

For example, the collection of sets { {1}, {2}, {3}, … } is pairwise disjoint. If {A_i} is a pairwise disjoint collection (containing at least two sets), then clearly its intersection is empty:

<math>\bigcap_{i\in I} A_i = \varnothing.\,</math>

However, the converse is not true: the intersection of the collection {{1, 2}, {2, 3}, {3, 1}} is empty, but the collection is not pairwise disjoint – in fact, there are no two disjoint sets in the collection.

A partition of a set X is any collection of non-empty subsets {A_i : i ∈ I} of X such that {A_i} are pairwise disjoint and

<math>\bigcup_{i\in I} A_i = X.\,</math>

• almost disjoint sets
• connectedness
• disjoint union
• disjoint-set data structure

cs:Disjunktní množiny de:Disjunkt eo:Vikipedio:Projekto matematiko/Disaj aroj ko:서로소 (집합론) is:Sundurlæg mengi it:Disgiunzione he:קבוצות זרות nl:Disjunct nn:Disjunkt sl:Disjunktni množici fi:Erilliset joukot fiu-vro:Ütidse osalda hulgaq