The main characteristic of a responsive web design is its ability to adapt to various screen sizes and orientations, providing an optimal viewing experience across devices. This design approach uses fluid grids, flexible images, and CSS media queries to ensure that web content is accessible, readable, and navigable on desktops, tablets, and smartphones. By maintaining usability and aesthetics regardless of the device, responsive design improves user engagement and retention, aligning with the increasing diversity of devices used to access the web.