Why a page is it not indexed by Google?

Some pages of my site do not appear in results of search engines. What could be causing this?

To determine whether a page is indexed or not, use in the query bar of the search engine, the command:

site: sitename.extension

The Google Webmaster Tools or those of Bing may also advise you on problems from the crawlers.

But the page may be present and hidden in the depths of the result pages. If it is completely absent, this may have several causes...

  1. If the page is new, it takes several days or weeks for it to be taken into account.
  2. The page is not known by search engines because no link allow them to access it. You must verify that it is accessible from another page.
  3. Indexing is prohibited by a meta-tag in the head section of the page.
    <meta name="robots" content="none">
    <meta name="robots" content="noindex">
  4. The canonical tag indicates that this page is located at another address. Check it not in your local code, but in the source code of the page online.
  5. The robots.txt file prohibits crawlers from accessing the page. Therefore it can not be indexed.
    It may take the form:
    User-Agent: *   
    Disallow: /dir/filename.html
  6. It is also possible that Google or another search engine decides not to index your website because the robots.txt file is malformed. See what is a robots.txt file.
  7. If a site is not indexed as a whole, the first thing to check is the robots.txt file to see if it does not block robots. Robots are blocked with a directive of the form Disallow: /
    They are not blocked if nothing follows Disallow:.
  8. Then see the list of errors to not commit in SEO.

How to make a page be indexed? The only way is to have a link to this page from another page already indexed. This may be an internal link or from another site.
It may also be a link in a sitemap.