Mobile-First Indexing is a search engine practice where the mobile version of a website is prioritized for indexing and ranking in search results. This approach reflects the growing trend of users accessing the web via smartphones and tablets. With mobile-first indexing, search engines like Google primarily use the content, structure, and performance of the mobile site to determine how pages should appear in search listings. Ensuring a responsive, mobile-friendly design is essential for maintaining visibility and ranking in search results.