Short answer
Responsive design is a single website built to reshape itself for whatever screen requests it, rather than maintaining a separate mobile site. Layout, type size, images and navigation respond to the viewport width, so one codebase serves a 360-pixel phone and a desktop monitor. Mobile behaviour is the default case, not an afterthought.
Also called: Mobile-friendly design, Adaptive layout
Google indexes the mobile version of a site, so what the phone renders is what search engines evaluate. That makes a common shortcut expensive: hiding content on small screens to tidy the layout removes it from the version being indexed. Content that matters has to be present on the phone, even if it is behind an accordion.
Passing a mobile-friendly check is a low bar that a site can clear while remaining unusable. The failures that cost money are specific — tap targets too close together, a checkout form demanding a 16-digit entry with no numeric keypad, a pricing table that scrolls sideways off the screen. Testing on a real mid-range Android handset on mobile data finds these in minutes, and a desktop browser resized to phone width never will.
Where this comes up in our work
Related terms
Core Web Vitals
Core Web Vitals are three metrics Google publishes to measure page experience: Largest Contentful Paint (loading, good at 2.5 seconds or under), Interaction to Next Paint (responsiveness, 200 milliseconds or under) and Cumulative Layout Shift (visual stability, 0.1 or under).
Progressive Web App (PWA)
A progressive web app is a website that behaves like an installed app: it can be added to a phone’s home screen, work offline through cached files, and receive push notifications, while still loading from a URL.
Local SEO
Local SEO is optimisation for searches with geographic intent — queries naming a city, or carrying implicit "near me" intent.
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