a building at a college or university used by students for recreation and other nonacademic activities.

I was on at the student union of the University of Central Lancashire today.

a club, society, or association formed by people with a common interest or purpose.

members of the Students' Union

a fabric made of two or more different yarns, typically cotton and linen or silk.

a part of a flag with an emblem symbolizing national union, typically occupying the upper corner next to the staff.

a pipe coupling.

The vertical gas line comes to a T joint below the union toward the bottom side of the water heater and is connected to the top vertical opening of the T joint.