Mobile First Indexing
Practice by search engines to crawl and rank a website mainly based on its mobile version instead of its desktop version.
Also known as: Mobile First Index, Mobile First Crawl
Mobile first indexing means that search engines primarily crawl and evaluate the mobile version of a website instead of using the desktop version as the reference. If the mobile version has fewer contents or worse performance, that directly hits ranking. Google introduced the approach gradually and now applies it almost everywhere.
What changes because of it
Content that is only visible on the desktop version no longer counts for ranking. Structured data, images and internal links also need to be fully present on the mobile version. Responsive sites barely have any extra work because mobile and desktop share the same code. Sites with separate m. subdomains usually benefit from moving to responsive or adaptive layouts.
What SEO teams pay attention to
Important factors are mobile load times, tap target size and spacing, readable font sizes without zoom and avoiding intrusive interstitials. Tools such as PageSpeed Insights or Lighthouse highlight common issues. Google Search Console also flags mobile usability errors.
In everyday SEO
In day to day SEO work mobile first indexing is the baseline every website has to meet. New content or features are designed for mobile first and then adapted for desktop as needed. Sites that work cleanly on mobile gain in ranking automatically, because the most important signals are measured there.