Responsive design in web development refers to an approach that ensures websites function optimally across various devices and screen sizes. This involves using flexible layouts, images, and CSS media queries to adapt the interface without sacrificing user experience. By prioritizing responsive design, developers can create sites that are accessible and visually appealing on desktops, tablets, and smartphones. This approach is critical for enhancing user engagement and improving search engine optimization (SEO) as it caters to increasing mobile users.