Index (: indexes or indices) most commonly refers to: * Index (publishing), an organized list of information in a publication * Web indexing, Internet indexing * An index, a key in an associative array Index may also refer to: Arts, entertainment, and mediaFictional entities * Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index'' * The Index, an item on the Halo Array in the ''Halo'' video game series Periodicals and news portals * '' Index Magazine'', a publication for art and culture * Index.hr, a Croatian online newspaper * index.hu, a Hungarian-language news and community portal * ''The Index'' (Kalamazoo College), a student newspaper * ''The Index'', an 1860s European propaganda journal created by Henry Hotze to support the Confederate States of America * '' Truman State University Index'', a student newspaper * ''The Index: A Weekly Paper Devoted to Free Religion'' (1870–1886), the semi-official periodical of the Fre ...

In finance, a stock market index or stock index is an index that measures the performance of a stock market, or of a subset of a stock market. It helps investors compare current stock price levels with past prices to calculate market performance. Two of the primary criteria of an index are that it is ''investable'' and ''transparent'': The methods of its construction are specified. Investors may be able to invest in a stock market index by buying an index fund, which is structured as either a mutual fund or an exchange-traded fund, and "tracks" an index. The difference between an index fund's performance and the index, if any, is called '' tracking error''. Types of indices by coverage Stock market indices may be classified and segmented by the set of underlying stocks included in the index, sometimes referred to as the "coverage". The underlying stocks are typically grouped together based on their underlying economics or underlying investor demand that the index is seek ...