Graph terminology: Are there terms for "a source or a sink" and for "neither source nor sink"? I often have to write something like "let $v$ be a vertex of the graph $G$ that is a sink or a source" or "that is neither a sink nor a source". Are there single-word terms for such notions? For example, for a function, "a point that is a maximum or a minimum" is called an extremum, and I would call "a point that is neither maximum nor minimum" a non-extremum. Are there such terms for digraphs?
Specifically, is there a term for a vertex of a digraph for which in-degree or out-degree is zero?
Is there a term for a vertex of a digraph that has both incoming and outgoing edges? I have seen people calling such vertices internal, but this seems not to be a common term (other than referring to non-leaf vertices in a tree, which is a completely different usage).
 A: There is some prior art, but nothing that will be universally recognized.
In the context of series-parallel digraphs, the source and sink are called the terminals of the graph. This is a slightly more specific case, but you might adopt it for general digraphs.
As you've mentioned, there's internal and its cousins interior and intermediate, which I expect to fill in the blank in a sentence along the lines of

In a network flow problem, flow conservation must hold at every ______ vertex.

But it's equally common even for people studying network flows to just suffer through the awkwardness of saying "at every vertex other than the source and sink". Unfortunately, internal vertices can also mean the non-endpoints of a path (which at least is not too far from this usage) or the vertices of a planar graph not on the external face of a given plane embedding.
Specifically in multi-commodity network flow, we might also call the intermediate nodes the transit nodes, but (in my opinion, at least) it makes less sense to use this terminology for a general digraph.
A: The nodes other than the source(s) or sink(s) are often called transshipment nodes.  See, for example, the second page of http://web.mit.edu/15.053/www/AMP-Chapter-08.pdf
