“Responsive Web Design” is a subset of a larger concept which is called “Adaptive Web Design”. When talking about responsive we refer to the layout only (Ethan Marcotte, fluid grids, flexible images & media queries).
“Adaptive design” on the other hand includes much more than just fluid layout. In practice it means the same thing as progressive enhancement, which means in short, that we aim to deliver the best possible experience to the widest possible audience. Also: “Adaptive design” shouldn’t be mixed with “Adaptive layout” which is a completely different thing.
Finally, “Adaptive layout” means a layout done by combining multiple fixed widths. Right now, I feel that Adaptive layout — when done right and when fixed widths are combined with fluid widths — is one form of responsive design and not just an alternative to it. Zeldman shared a bit similar idea in his blog post from 2011.
[This article was written by Viljami Salminen and can be found here.]