Hi Bhante, very nice essay!
In philosophy of science, instrumentalism refers to a viewpoint about scientfic theories. Namely: scientific theories only predict what will be the outcomes observed when we make measurements with instruments. No further ontological commitments are made.
So contrast this with the normal Newtonian picture. In Newtonian mechanics, you have some trajectory that describes the motion of a ball or particle through space. You can predict where the ball will land. But there is also the ontological idea that the ball really is “out there” going through space according to the trajectory predicted by the equations of motion.
This may seem sort of a strange viewpoint, but it is more persuasive in the context of quantum mechanics. It is pretty hard to say exactly just what is happening in between (quantum) measurements. The wave function really only describes the probabilities of measurements. The Copenahagen interpretation posits that the wave function is ontologically real and that measurements “collapse” it. Instrumentalism just refuses to even ask the question.
I think in general the term instrumentalism might be used more broadly about ideas in general, but the above viewpoint is how I originally learned about it in coursework.