index

American  

[in-deks] / ˈɪn dɛks /

noun

• indexes,
  plural
• indices
  plural

1. (in a nonfiction book, monograph, etc.) a more or less detailed alphabetical listing of names, places, and topics along with the numbers of the pages on which they are mentioned or discussed, usually included in or constituting the back matter.

2. a sequential arrangement of material, especially in alphabetical or numerical order.

3. something used or serving to point out; a sign, token, or indication.

   a true index of his character.

4. something that directs attention to some fact, condition, etc.; a guiding principle.

5. a pointer or indicator in a scientific instrument.

6. a piece of wood, metal, or the like, serving as a pointer or indicator.

7. Digital Technology.

   1. a value that identifies and is used to locate a particular element within a data array or table.

   2. a reference table that contains the keys or references needed to address data items.

   3. the part of a search engine where website addresses are cataloged with their associated keywords in order to quickly retrieve the most relevant web pages when a user submits a search query.

8. Also called fist, hand. Printing. a sign in the shape of a hand with extended index finger, used to point out a particular note, paragraph, etc.

9. a light, smooth cardboard stock.

10. the forefinger.

11. a number or formula expressing some property, ratio, etc., of something indicated.

    index of growth; index of intelligence.

12. Statistics. index number.

13. Economics. price index.

14. Algebra.

    1. an exponent.

    2. the integer n in a radical defining the n- th root: ∛ is a radical having index three.

    3. a subscript or superscript indicating the position of an object in a series of similar objects, as the subscripts 1, 2, and 3 in the series x 1 , x 2 , x 3 .

    4. winding number.

15. Horology. a leverlike regulator for a hairspring.

16. (initial capital letter)

    1. Index Librorum Prohibitorum.

    2. Index Expurgatorius.

17. (usually initial capital letter) any list of forbidden or otherwise restricted material deemed morally or politically harmful by authorities.

    an Index of disapproved books relating to Communism.

18. Optics. index of refraction.

19. Obsolete.

    1. a table of contents.

    2. a preface or prologue.

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verb (used with object)

• indexes,
  present (3rd person singular)
• indexed,
  past participle,  past
• indexing
  present participle

1. to provide with an index, as a book.

2. to enter in an index, as a name or topic.

3. to serve to indicate.

   warm breezes indexing the approach of spring.

4. to place (a book) on an official list as politically or morally harmful.

   The commissar insisted on indexing the book.

5. to rotate (work) on a milling machine in order to repeat the milling operation at a new position.

6. Economics. to adjust (wages, taxes, etc.) automatically according to changes in the cost-of-living level or another economic indicator, especially to offset inflation.

7. Digital Technology. (of a search engine) to catalog (a website) using keywords.

   Search engines use automated algorithms to index billions of web pages, but that still accounts for only a fraction of the content on the internet.

index British  

/ ˈɪndɛks /

noun

1. an alphabetical list of persons, places, subjects, etc, mentioned in the text of a printed work, usually at the back, and indicating where in the work they are referred to

2. See thumb index

3. library science a systematic list of book titles or author's names, giving cross-references and the location of each book; catalogue

4. an indication, sign, or token

5. a pointer, needle, or other indicator, as on an instrument

6. maths

   1. another name for exponent

   2. a number or variable placed as a superscript to the left of a radical sign indicating by its value the root to be extracted, as in ³√8 = 2

   3. a subscript or superscript to the right of a variable to express a set of variables, as in using x i for x 1 , x 2 , x 3 , etc

7. a numerical scale by means of which variables, such as levels of the cost of living, can be compared with each other or with some base number

8. a number or ratio indicating a specific characteristic, property, etc

   refractive index

9. Also called: fist. a printer's mark ( ) used to indicate notes, paragraphs, etc

10. obsolete a table of contents or preface

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verb

1. to put an index in (a book)

2. to enter (a word, item, etc) in an index

3. to point out; indicate

4. to index-link

5. to move (a machine or a workpiece held in a machine tool) so that one particular operation will be repeated at certain defined intervals

index Cultural  

1. An alphabetical list of subjects treated in a book. It usually appears at the end of the book and identifies page numbers on which information about each subject appears.

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Usage

Plural word for index The plural form of index is either indices, pronounced [ in-duh-seez ], or indexes. Indices is more widely used. The plurals of several other singular words that end in -ex or -ix are also formed this way, including appendix/appendices, cortex/cortices, and codex/codices. Irregular plurals that are formed like indices derive directly from their original pluralization in Latin. However, the standard English plural -es is often also acceptable for these terms, as in appendixes.

Other Word Forms

Derived Forms

• indexer
  noun
• overindexing
  noun
• overindex
  verb (used with object)
• reindex
  verb (used with object)
• indexable
  adjective
• indexical
  adjective
• indexless
  adjective
• nonindexed
  adjective
• unindexed
  adjective
• well-indexed
  adjective
• indexically
  adverb

Inflected Forms

Nouns

• indexes
  plural
• indices
  plural

Participles

• indexing
  present
• indexed
  past

Conjugated Forms

Present

• index
  simple
• indexes
  simple
• have indexed
  perfect
• has indexed
  perfect
• am indexing
  progressive
• are indexing
  progressive
• is indexing
  progressive
• have been indexing
  perfect progressive
• has been indexing
  perfect progressive

Past

• indexed
  simple
• had indexed
  perfect
• was indexing
  progressive
• were indexing
  progressive
• had been indexing
  perfect progressive

Future

• will index
  simple
• will have indexed
  perfect
• will be indexing
  progressive
• will have been indexing
  perfect progressive

Etymology

Origin of index

First recorded in 1350–1400; Middle English, from Latin: “informer, pointer,” equivalent to in- in- ² + -dec- (combining form of dic-, “show, declare, indicate,” akin to teach; see indicate) + -s nominative singular ending;

Explanation

Can’t find any information on the yellow-eyed junco in your guide to rare birds? Check out your bird book's index. It’s an alphabetical list in the back of the book that catalogs all of the subjects covered by page number. In financial terms, an index is a list of stocks that are traded on the stock market, such as the Dow Jones Indexes. “Index cards” were once used to catalog books in a library, but now they’re mostly used to prepare for a test or a presentation, thanks to computers. As a verb, index describes the action of creating an index, such as, “I’m going to index my photo collection on my computer, so I can find what I’m looking for.”

Example Sentences

See Examples For:

index indexes indices indexed indexing

The index of new orders climbed 1.6 points last month to a robust 55.3%.

From MarketWatch ● Oct. 1, 2026

The S&P Global Canada manufacturing purchasing managers index eased to 51.5 in September, the lowest reading since March and a signal of only modest growth.

From The Wall Street Journal ● Oct. 1, 2026

How a “core and explore” approach, with index funds at the core, could give investors an edge.

From Barron's ● Oct. 1, 2026

Last month, Nike was ejected from the S&P 100 stock market index of the biggest blue-chip firms in the US.

From BBC ● Oct. 1, 2026

I sat down, tipped my chair back and started to thumb through the index of one of the books.

From "Liar, Liar" by Gary Paulsen

The case for letting fund managers charge performance fees, where they get paid extra if their funds beat the market indexes or a benchmark, is very easy to make.

From MarketWatch ● Oct. 1, 2026

Here’s another chart that shows just how out of step major indexes like the S&P 500 are with the average stock listed on the New York Stock Exchange.

From MarketWatch ● Oct. 1, 2026

“As long as the AI trade is working, given the makeup of the big indexes, you can have a positive quarter,” said Ross Mayfield, investment strategist at Baird.

From The Wall Street Journal ● Sep. 30, 2026

Even if some company valuations are stretched, increases in stock prices are broader than a couple of years ago when indexes were driven by the so-called Magnificent Seven.

From The Wall Street Journal ● Sep. 30, 2026

I found some books on experimental psychology and leafed through them slowly, then checked the indexes and bibliographies.

From "The Chosen" by Chaim Potok

"Any change in the AI trade will have big ramifications for US indices," she warned.

From Barron's ● Sep. 14, 2026

“European indices are more vulnerable to changes in energy prices and the global economic outlook, as they are more cyclical than their international peers,” Swissquote’s Ipek Ozkardeskaya wrote in a research note.

From The Wall Street Journal ● Sep. 9, 2026

That’s according to Citi research analyst David Chew, who in a note published Monday analyses current investor positioning in the main U.S. equity indices.

From MarketWatch ● Aug. 11, 2026

Temperatures climbed into the 90s with heat indices exceeding 100 degrees, forcing organizers to scale back outdoor programming, expand cooling stations and urge visitors to take frequent breaks.

From Salon ● Jul. 4, 2026

Activities or illnesses whose safety indices are greater than 6 should be considered quite safe, being equivalent to less than one chance in a million per year.

From "Innumeracy: Mathematical Illiteracy and Its Consequences" by John Allen Paulos

It now hits far fewer taxpayers as the threshold matches the federal exemption amount, which is $15 million for 2026 and is indexed for inflation.

From The Wall Street Journal ● Aug. 8, 2026

"We give site owners clear controls to decide whether pages can be crawled or indexed, and we always respect those directives."

From BBC ● Jul. 27, 2026

Three hundred years later, Gray’s Anatomy by Henry Gray reinforced the impression that the body had finally been catalogued, indexed and neatly organized – a system mapped and fully explained.

From Science Daily ● Jun. 21, 2026

He’s tried Claude Code before but prefers Cursor’s indexed codebase, which allows users to query a multi-file project in natural language and find relevant code.

From MarketWatch ● Jun. 19, 2026

Before it could go to the Marshall Library, everything had to be indexed.

From "The Woman All Spies Fear" by Amy Butler Greenfield

Among them: why indexing beats even the most talented stock pickers; the biggest mistake individual investors are making right now, and what a young investor can learn from a lifetime spent studying markets.

From MarketWatch ● Jul. 29, 2026

As the search indexing of the chat logs is no longer occurring, it is likely Anthropic used available tools to quickly block the chat log links from search results.

From BBC ● Jul. 27, 2026

One such strategy, direct indexing, involves buying individual securities that track the performance of an index—such as the S&P 500 or the Schwab 1000—while also generating losses that can be used to reduce capital-gains taxes.

From The Wall Street Journal ● Jul. 10, 2026

Also until 2020, for retail investors to buy DFA’s funds they had to go through fee-charging advisors, which would seem to violate the code of indexing.

From Barron's ● Jun. 18, 2026

Instead she stayed put and kept indexing the collection, determined to preserve William’s place in history.

From "The Woman All Spies Fear" by Amy Butler Greenfield

Related Words

• indicator
• ratio