What is a union?

A union is the joining of two or more things.

Answer

A union is an organized labor association.

As in the union formed by the United States, as in an organized labor union, a sexual union also implies more than joining. A union is as much a blending as a joining. In mathematics a union is the collection of distinct elements in the blended part of two sets. It is fair to say that the object of all unions is unity.