Extended definition
The process by which Google adds a page to its index (the database it answers searches from). If the page isn't indexed, it doesn't appear in results, no matter what other signals you have.
Context and application
Indexing isn’t guaranteed even if the page is crawlable. Google evaluates content quality and decides whether it deserves indexing. On sites with lots of thin content (under 300 words, repetitive), Google may choose not to index pages even if you want it to. Problem indicator: lots of “Discovered, not indexed” or “Crawled, not indexed” in Search Console. Solution: improve content quality or remove thin pages (consolidation).
In practice
check Google Search Console "Coverage" → "Indexed" to see how many pages are indexed.
Related terms
Crawl, Noindex, Sitemap.