In software, module A can call onto another module B to do stuff. Now I put a third module C in between A and B, whose job it is to catch calls from module A, possibly divert them, and otherwise pass them through to module B. What do you call this interposed C?
I'm pretty sure there is a term for this in mechanics/electronics/carpentry, and hence I'm not looking for a software-specific term. (Before anyone asks: a proxy is a hyponym of the hyperonym I'm looking for. An interface is also not the right term, since that denotes a thin layer.)
For some reason, my mind keeps going to "shunt" (an electrical term), but I don't think that's what I need. A "wedge" has no ability to pass things through, so that's not right.