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

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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;

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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 Institute for Supply Management’s purchasing managers index for services providers was 54.5 in October, compared with 54.9 in September.

From The Wall Street Journal ● Nov. 4, 2026

The value reached 86.4 on an index where 100 equals the value in 2005.

From BBC ● Nov. 4, 2026

A handful of tech and tech-adjacent stocks have amassed unparalleled influence over the direction of the index.

From MarketWatch ● Nov. 4, 2026

“There’s always human judgment—every index is a set of rules.”

From The Wall Street Journal ● Nov. 4, 2026

I wrote the poem on an index card and memorized it while I cleaned the gas company.

From "Breaking Through" by Francisco Jiménez

Oil prices saw a steep drop in response, while U.S. benchmark stock indexes rallied on Wednesday.

From MarketWatch ● Nov. 4, 2026

In May, when Wall Street's major stock indexes were near historic highs, economists at the New York Federal Reserve warned they were seeing a "remarkable increase" in food insecurity in the world's richest country.

From Barron's ● Nov. 4, 2026

U.S. stock-market indexes were mixed as investors prepared to digest midterm election results.

From The Wall Street Journal ● Nov. 3, 2026

The elections have sparked hedging in some corners of the market, including cryptocurrencies, AI-related stocks and major indexes such as the S&P 500.

From The Wall Street Journal ● Nov. 2, 2026

The warden’s library is a big dark room with heavy red drapes drawn closed and floor-to-ceiling books—the kind of official volumes with thick indexes Natalie likes.

From "Al Capone Does My Shirts" by Gennifer Choldenko

A modest 2% increase in ratings revenue to $1.27 billion was offset by robust 20% growth in indices revenue to $554 million.

From The Wall Street Journal ● Oct. 27, 2026

However, that exposure will grow if Anthropic goes public because its shares would become included in several stock indices, including the Russell 3000, which New York City tracks for its own investments.

From Barron's ● Oct. 15, 2026

India's benchmark equity indices Sensex and Nifty fell with investors weighing the implications of higher borrowing costs for consumption and corporate investment.

From BBC ● Oct. 7, 2026

The positive trend in U.S. earning revisions in 2026 has therefore been one explanation why major U.S. stock indices have been remarkably resilient so far, despite adverse moves in energy prices, inflation and bond markets.

From MarketWatch ● Sep. 28, 2026

He's only in the door three seconds before he starts spouting some theory about spectral analysis and the refractive indices of protein substances, and he's carrying a big box full of books and papers.

From "Things Not Seen" by Andrew Clements

Her son-in-law helped her put $40,000 into an Aviva indexed annuity.

From MarketWatch ● Oct. 20, 2026

The Chicago Fed paper found that private-equity-owned insurers more than doubled their share of the indexed annuity market, from 16% to 33%.

From MarketWatch ● Oct. 20, 2026

“Rates were going up, but they indexed really close to inflation.”

From Los Angeles Times ● Oct. 17, 2026

Yet sales of indexed annuities have climbed to $128 billion in 2025, up from $96 billion in 2023.

From The Wall Street Journal ● Oct. 11, 2026

A simple task be-cause Brother Leon insisted that all kinds of cross- indexed lists be kept so that you could tell at a glance just where students stood.

From "The Chocolate War" by Robert Cormier

One such strategy, direct indexing, involves buying individual securities that track the performance of an index while also generating losses that can be used to reduce capital-gains taxes.

From The Wall Street Journal ● Aug. 23, 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

But his approach is direct indexing — he uses Frec.com, one of Social Leverage’s “big investments” — so he can tailor an index like the S&P 500 to his preferences.

From MarketWatch ● Jul. 21, 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

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Related Words

• indicator
• ratio

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