The Index

"An index is a list of data, such as group of files or database entries. It is typically saved in a plain-text format that can be quickly scanned by a search algorithm. This significantly speeds up searching and sorting operations on data referenced by the index. Indexes often include information about each item in the list, such as metadata or keywords, that allows the data to be searched via the index instead of reading through each file individually."

—TechTerms.com