Responsive design in web development refers to a design approach that ensures websites are optimized for a variety of screen sizes and devices. It employs fluid grids, flexible images, and media queries to create layouts that adjust according to the user's device, whether it's a smartphone, tablet, or desktop computer. This adaptability not only enhances user experience by providing consistent navigation and accessibility but also plays a crucial role in SEO, as search engines prioritize mobile-friendly sites. Ultimately, responsive design is essential in today’s digital landscape, where diverse devices are widely used to access online content.