An added part that extends a system without altering it. A plugin belongs to the host and is not of it.
From plug (Dutch plugge, a stopper, bung) and in (Old English in); a 20th-century electrical metaphor for an added part that extends a system by being inserted into it, belonging to the host without becoming part of it.
An added part that extends a system without altering it. It belongs to the host it is inserted into, without ever becoming part of it.