Incidentally, it is useful to avoid confusing backpressure with scavenging. Scavenging is good and backpressure is bad, but the two are often confused.
https://vpexhaust.com/exhaust-back-pressure-a-myth/
https://youtu.be/jjPeP_Nn2B4
This is fact, not opinion. It may be incredibly useful 
Getting into the deep weeds on engine exhaust science is probably beyond you and me.
You can call it whatever you want, but in my experience on my stock 22RE an opened exhaust that reduced the backpressure too much in the exhaust system definitely decreased the low-end torque. During my face-to-face conversation with a Doug Thorley engineer for about 1 hour, he used the word "backpressure" at least a dozen times and he was specifically referring to the DT Tri-Y design. He also referred to scavenging, pulse waves, velocity, and exhaust gas temperature and density.
So perhaps it is just conceptual semantics, and a common word used when discussing exhaust systems, albeit by the lesser educated?.
The "pressure" part of backpressure logically refers to and perhaps describes affect of the exhaust gas density. More dense (cooler), the slower it moves... less flow... less power.
Gnarls.... of course I could be full of backpressure!
