Considering the terms, elastic, responsive and adaptive, when designing for web sites, many people are using them interchangeably. To help communicate with project members, it would be nice to define theses terms.
It would be a good practice to keep web designs to be elastic to screen size with a fluid layout, responsive to viewport with various layouts, and adaptive to device capacities with various layouts and/or contents.
Pages with a fluid layout are capable of rearranging elements properly when the screen or window size changes; a fluid layout defines element size in terms of percentage instead of pixels.
Pages are responsive by changing layouts when the viewport (or screen/window size) changes and the current layout cannot deliver and present contents effectively.
Pages are adaptive by changing layouts and selecting contents according to the device capabilities; screen or window size is just one of the many device capabilities to consider. Flash animations, for example, may be delivered on desktops but filtered or hidden on mobile devices.