The expression \( A \cup B \) (read as "A union B") represents the union of two sets, \( A \) and \( B \). The union of two sets includes all the elements that are in either set \( A \), set \( B \), or in both.
In set notation, this can be expressed as:
\[ A \cup B = { x \mid x \in A \text{ or } x \in B } \]
If you have specific elements in mind for sets \( A \) and \( B \), or if you would like to explore properties of unions or examples, feel free to provide more details!