Term to describe a subgraph of a digraph where no arcs go into it from outside

Question

What's the shortest way to describe a subgraph of a digraph where, in the original graph, no arcs go into it from outside? Is there a term for this?

Note that it might or might not be connected.

Here's an example:

A real life example is an intranet. It can have hyperlinks to the WWW, but the WWW won't have hyperlinks to it.

Answer 1

I suggest a source subgraph, because a source is a node with no incoming edge.