The act of uniting into a new whole.
In math, a union is the combination of two or more sets into one larger set that includes all the elements from each of them. If something is in either set—or in both—it’s included in the union.
We use the symbol ∪ (a rounded U) to show a union.
For example:
-
If Set A = {1, 2, 3} and Set B = {3, 4, 5}, the union is:
-
A ∪ B = {1, 2, 3, 4, 5}
-
We only list each element once, even if it appears in both sets.
Unions are useful in sorting, data organization, probability, and understanding relationships between different groups.
When Do Students Learn About Unions?
Students begin learning about unions as part of their introduction to sets and how they relate to one another.
Grades 4–5 – Introduction to Sets and Unions
Students explore sets, elements, and begin learning how unions combine different groups.
Grades 6+ – Applying Unions in Probability and Algebra
Students use unions to solve problems involving data, probability, and relationships between multiple sets.
