Short definition
Responsive design is a website-building approach where layout, type, and images adapt fluidly to any screen size from a 320px phone to a 4K monitor.
In depth
Responsive design uses CSS media queries, flexible grids, and fluid images so a single codebase serves every device. It is now the baseline expectation; Google indexes the mobile version primarily and penalizes pages that are not mobile-friendly. Beyond technical compliance, true responsive design considers thumb zones, mobile reading patterns, and tap targets, not just visual scaling.
Example
A roofing site is rebuilt with mobile-first responsive design. The mobile-only conversion rate doubles because every CTA finally lives inside the thumb-friendly bottom third.