Responsive design refers to an approach in web development that ensures a website adapts fluidly to various screen sizes and devices. This design strategy optimizes user experience by providing a consistent interface whether the site is accessed on a desktop, tablet, or smartphone. Responsive design incorporates flexible grids, images, and CSS media queries to adjust layout and content dynamically based on the user's device. By enhancing accessibility and usability, responsive design is crucial for modern websites that reach diverse audiences.