Isn't it technically more about using CSS3 and JS to adapt to the browser width and/or capabilities, while the HTML otherwise remains the same or similar?
In contrast to browser detection and/or loading a completely different HTML representation, with or without a completely separate URL.
Yup. And those things aren't nearly as complicated as people make them out to be. The more "things" you add to a layer, the more you need to tweak those things and more the likelyhood of something breaking, not looking quite right or just plain failing.
It's actually possible to have one stylesheet dictate the vast majority of layout needs. Just a matter of people willing to sit down and take a patient look at what it is they're actually trying to accomplish (vs, what they can throw at the browser).
Well, according to Ethan Marcotte, RWD has three key components: media queries, a fluid grid, and flexible images. What you describe is closer to "adaptive" design - the design adapts to different resolutions but isn't necessarily fluid. RWD is a subset of that.
In contrast to browser detection and/or loading a completely different HTML representation, with or without a completely separate URL.