Responsive Design is a web design approach that creates websites which automatically adjust their layout, images, and content to provide optimal viewing experiences across different devices and screen sizes.

Key principles of responsive design:

• Fluid grids: Using relative units (percentages) instead of fixed pixels
• Flexible images: Images that scale within their containing elements
• Media queries: CSS rules that apply styles based on device characteristics
• Mobile-first approach: Designing for mobile devices first, then scaling up

Responsive design is essential for SEO, as Google uses mobile-first indexing.