An index (n) (pl. indices) in SharePoint terms generally refers to a listing of all content and its attributes that the SharePoint search engine logs for efficient searching when a user types in a search query in a search box in SharePoint Online or on-prem. On a periodic basis, the search engine within SharePoint will index (v) the content stored within it, adding new content, updating existing content, and removing deleted content from the index, thereby producing the best results possible on the SharePoint search results page. This term is also sometimes known as crawling or spidering; while these concepts are not identical to indexing, they are related closely enough for the purposes here to be considered synonyms.
What gets indexed
SharePoint 2013 and later versions (including Online) indexes the metadata and the content of a file, item, library, or list unless the system is specifically set up to not index the content. That includes full text of documents. This includes PDFs that are either created natively via software that can save or print to PDF (like Word, Excel, etc.) or a scanned PDF that has been OCR'ed.
There are two types of crawls: a complete crawl and an incremental crawl. A complete crawl occurs when the search engine reviews the entire system and indexes it as if it had never seen it before. An incremental crawl looks at only the changes that occurred since the last crawl (whether it was complete or incremental). Incremental crawls generally take significantly less time than complete crawls because there is usually drastically less content to review.
For example, on a SharePoint system that has 150 GB of content, a full crawl would review all 150 GB of content; an incremental crawl would only index the, say, 500 MB worth of changes made by users since the last incremental crawl. Reviewing and indexing 500 MB is significantly less strenuous than performing the same operations on 150 GB.
When indexing occurs
Changes that occur in SharePoint won't be noted in search results until the index is updated. So, any edits to files, uploads of new files, etc. won't be displayed in search results until they've been indexed.
SharePoint on-prem indexes occur at the discretion of the system administrator. They decide what gets crawled and how often. That means they can change the settings on the index if users determine indexing isn't happening often enough. Generally speaking, many systems perform an incremental index each night, after hours, so the search results won't change on users during the day while they depend on the system. That said, if the system is small, indexing may occur multiple times per day; if it's very large, indexing may occur every other day due to the system resources needed to perform the operations.
SharePoint Online indexes more frequently, though Microsoft has yet to identify and official schedule. Generally speaking, incremental indexing occurs every four to six hours in SPO, though actual use indicates that indexing occurs significantly more frequently. Complete indexing does not occur in SPO and cannot be forced.
Forcing a re-index
If you want to see the updated content included in the search index immediately, a site owner can force an indexing within their site. See article below under Further reading.
- Technet: Overview of the search schema in SharePoint Server 2013
- Microsoft: Manually request crawling and re-indexing of a site, a library or a list (article for SPO, but applies to SP 2013 and 2016)