A design for a web site to accommodate varying sizes of screens or windows.
A responsive site design is a set of designs or layouts for a web site that will accommodate various devices, window or screen sizes, or browser widths providing a good user experience across all interfaces.
While there is an increased expense to create a site with a responsive design, providing specific layouts for different mobile devices can provide a better experience for users, particularly those on handheld devices like smart phones and tablets. A responsive design will turn off some elements on narrower screens, and move other elements into series after one another rather than being beside each other on the screen all at once. However, it should be noted that a responsive design changes the presentation of the page, and may disorient users used to the presentation of your site on a desktop or laptop computer - sometimes offering a mobile specific version with the option to switch to a desktop layout is a better solution.