Adapter

An adapter typically goes from socket-weld to threaded, although exotic things like socket-weld to bulkhead are available.
In the threaded application variety, it comes in 2 types: "male" and "female". Males have threads on the outside, females have threads on the inside. This allows the male to be screwed into the female fitting, forming a non-permanent joint.

Because of this usefully property, it is often used in male-female pairs to fulfill the applications of a coupler (connecting pipes), while keeping the pipes separatable.