Google's practice of using the mobile version of a page for indexing and ranking, rather than the desktop version.
Mobile-first indexing means that Google primarily uses the mobile version of a website's content for indexing and ranking. This shift, completed across all sites by late 2023, reflects the reality that the majority of Google searches now come from mobile devices.
With mobile-first indexing, if your mobile page has less content, fewer links, or different structured data than your desktop version, the mobile version is what Google evaluates. This makes responsive design (a single page that adapts to all screen sizes) the safest approach.
Sites that serve a stripped-down mobile experience with less content or fewer features than desktop are at a disadvantage under mobile-first indexing. Google only sees the mobile version, so any content hidden on mobile is effectively invisible to the search engine.
Mobile performance also matters more under this paradigm. Core Web Vitals are measured on the mobile version, and mobile page speed directly impacts rankings.
A site uses a separate mobile domain (m.example.com) that shows only product images and prices, while the desktop version includes detailed descriptions, reviews, and FAQ sections. Under mobile-first indexing, Google only sees the sparse mobile content, and the site loses rankings for queries that the desktop content would have matched.
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