A union is similar to a coupling, except it is designed to allow quick and convenient disconnection of pipes for maintenance or fixture replacement. While a coupling would require being able to rotate the connected pipes, a union provides a simple transition, allowing easy connection or disconnection at any future time. The union is made in three parts, when ends are joined, the nut provides the necessary pressure to seal the joint.
Not for use with fresh water plumbing, normally used for gas / air / oil /sprinkler / heating applications.
These all have British Standard Pipe Threads (see bspt link at bottom of page for more info on plumbing thread sizes).