In mathematics, "u" usually stands for the symbol "union". A union is a set of all elements in two or more sets, and repeated elements are only calculated once. For example, if set A contains {1, 2,3} and set B contains {2,3,4}, the union of A and B is {1, 2,3,4}.

The symbol "u" comes from the Latin word "union", which means "union" or "merger". In the fields of mathematics such as set theory, probability theory and logic, it is not a basic and important concept, which helps to understand the relationship and combination between different sets.