An index is a concise and organized list or indicator that helps locate, categorize, or measure things. It can refer to financial market indicators, search engine databases, data structures in databases, or alphabetical lists in books. In essence, an index simplifies access to information or gauges the performance of a specific group.
An index is a copy of selected columns of data, from a table, that is designed to enable very efficient search. An index normally includes a “key” or direct link to the original row of data from which it was copied, to allow the complete row to be retrieved efficiently.