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

This is an investment-grade index of bonds paying interest that is exempt from federal income taxes.

From MarketWatch ● Sep. 11, 2026

Poring over the CPI numbers, many economists now think that the core version of the PCE price index rose by 0.3% last month, a pace many Fed watchers have speculated could trigger a rate hike.

From The Wall Street Journal ● Sep. 11, 2026

For the study, Varady recruited adults with Type 1 diabetes who were also classified as obese according to the Centers for Disease Control and Prevention's body mass index.

From Science Daily ● Sep. 11, 2026

That comes after the release later Friday of the US consumer price index, with a strong figure likely to force the policymakers to hike.

From Barron's ● Sep. 11, 2026

By the middle of 2005, over a period in which the broad stock market index had fallen by 6.84 percent, Burry’s fund was up 242 percent and he was turning away investors.

From "The Big Short" by Michael Lewis

After all three indexes on Wall Street ended deep in the red, along with Europe, Asia followed suit.

From Barron's ● Sep. 11, 2026

It was the third straight loss for the U.S. indexes.

From The Wall Street Journal ● Sep. 9, 2026

The prospect of borrowing costs heading up is weighing on equities, with all three main indexes dropping on Wall Street.

From Barron's ● Sep. 9, 2026

On the other hand, when energy has done badly it has usually been while stock indexes, and bonds, have done well.

From MarketWatch ● Sep. 9, 2026

We checked indexes and even leafed through some of the books page by page to be sure.

From "Kindred" by Octavia Butler

From the conclusions drawn by Bank of America’s proprietary “bubble risk indicator,” analysts remain relatively untroubled about U.S. stock indices and say equities will quickly bounce back from any pullback.

From MarketWatch ● Sep. 10, 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

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

A massive tech selloff roiled Korean stock indices over the past month, as the severity of wartime shortages became clear.

From Slate ● Jul. 2, 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

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

According to the latest data from LIMRA, a trade organization for the insurance industry, after a record-setting 2025, sales of indexed universal life insurance continued to surge in 2026.

From MarketWatch ● Jun. 16, 2026

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

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

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

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