Essentially, not much just different threads or port configuration. Think of it as an adapter “adapts”. An adapter uses at least two different thread sizes or connection types such as luer connections in the adapter regardless of the tubing size or type. A union brings different tubing together regardless of size or type but uses the same threads on all ports.

Adapters connect two different types of thread/port configurations, while unions have the same thread/port configuration on both sides. Both Adapters and Unions can connect different types or ID of tubing.

A union that connects two different size tubing is called a reducing union.