Definition - Search Engine Index / Indexing

In search engine optimization, the term "index" is both a noun and a verb. A search engine indexes your site, meaning it adds your site to its index.

Basically, an index is a huge database of websites that a search engine has found, evaluated and indexed. This will make more sense if we talk about the basics of how search engines work:

A search engine grows its index / database through the use of robotic web crawlers (often called spiders). When the search engine spiders find a new website (or one that they're revisiting), they crawl through the site to index as many pages as they can reach.

If Google does not have a particular web page or site in its index, nobody will be able to find that page by searching in Google ... because they're actually searching Google's database as opposed to searching the entire web. Make sense?

This is why you hear people say things like: "I need help getting my site indexed" or "Hey, only half of my pages have been indexed!"

You can increase the number of web pages that get indexed by having a well-organized site with a good navigation structure, and also by having plenty of inbound links to your internal pages (i.e. deep links).