Index: An index is a collection of data established as a bank, allowing computers to search through the information to find a match to a user-fed query. Some of the larger indexes can have billions of documents in their catalogs. These are often utilized by search engines. However, an index can also refer to the root of a folder on a web server.
Search engines operate using reverse indexes, taking words submitted by the user and returning results based on matching or relevant data. Stemming and semantic analyses allow search engines to return near matches based on these factors.
An index may also be called a catalog. Both terms are likely familiar to anyone who has every searched for a book or text in a library, whether using the older key-card catalogs or the modern computer indexes. Unsurprisingly, the two uses of both terms is connected to this earlier usage.