Definition
Mobile-first indexing is Google's policy (since 2019) of primarily using the mobile version of a page for indexing and ranking. AI engines mirror this: most LLM crawlers identify as mobile UAs or render at mobile viewport. Pages where desktop has rich content but mobile is stripped/hidden suffer in AEO because the bot only sees the mobile-stripped version.
Desktop has author bio + 5 related articles in sidebar. Mobile collapses sidebar with display:none. AI engines see neither.
How to optimize
Ensure mobile and desktop content parity: use responsive design (CSS), not user-agent forking. Test with Google's mobile-friendly tool + Lighthouse mobile audit.