To my ears, "Physically Based Rendering" does sound a little stilted. I don't immediately have an intuitive grasp of what it means based on words alone.

It doesn't sound like it means "physics based rendering", which would mean "a rendering process that uses physics at its core."

Rather, the attachment I have to the word "physically" is more along the lines of "metaphorical vs. physical" (i.e. "concrete", "real", "observable"). So along those lines, my natural understanding of "Physically based" rendering would be a process not based on abstract mathematical principles, but one that tries to take the physical properties of objects into account. So, maybe to render a rock you'd think about what it's made of, if moss grows on it, etc.

(I don't know if that's actually what PBR means, just that's what it sounds like to me as a native speaker.)