Indexing
Indexing is the stage where a fetched page is rendered, parsed, tokenised and written into a search engine's index as a retrievable record. Being crawled does not guarantee being indexed: engines select what to store, and discard duplicates, thin pages and pages carrying a noindex directive.
Indexing sits between crawling and ranking, and it is where most unexplained
visibility problems actually live. A page can be fetched successfully, return a
clean 200, and still never be stored.
The stage does several things in sequence: render the page so JavaScript-built content is visible, parse the HTML into a document tree, extract and normalise the text into tokens, decide which URL in a duplicate cluster is canonical, and write the terms into the inverted index alongside the per-document data ranking will need later.
The common reasons a crawled page is not indexed are worth memorising: it is a
near-duplicate of something already stored, it carries noindex, its content is
thin or auto-generated, its rendering failed and it appears empty, or it is a
soft 404 that returns 200 while saying the content is missing.
The Page Indexing report in Google Search Console names the specific reason per URL, which makes this the one stage where the diagnosis is usually handed to you.