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

Here they plot the rise in the 30-year yield versus the S&P 500 and the energy component of the Bloomberg commodity index, on an impressive triple Y-axis.

From MarketWatch ● Aug. 28, 2026

In 2026, fast trading is almost the norm, with more than 3 million daily trades in S&P 500 index option contracts that expire the same day.

From The Wall Street Journal ● Aug. 28, 2026

The researchers suggest creating an "underwear index" that could make soil processes easier for the public to understand while drawing attention to the importance of protecting healthy soils.

From Science Daily ● Aug. 28, 2026

Wall Street worked itself into a lather over the idea that the Fed could adopt a more forgiving inflation measure than the personal-consumption-expenditure index the central bank tracks.

From The Wall Street Journal ● Aug. 28, 2026

She began to dot the paste onto each spider bite with her index finger.

From "Beasts of Prey" by Ayana Gray

Wall Street had provided a positive lead, with all three main indexes rising, though the gains were based entirely on the tech sector, which was the only one of 11 to advance.

From Barron's ● Aug. 28, 2026

Treasury yields, a more modest rise in longer-term bond yields and declines in major stock indexes.

From The Wall Street Journal ● Aug. 28, 2026

—European stock indexes largely rose at the open.

From The Wall Street Journal ● Aug. 26, 2026

Stock traders were even less moved by the news, with the major indexes of the biggest companies listed in the US, Europe, and Asia barely shifting.

From BBC ● Aug. 25, 2026

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

From "Kindred" by Octavia Butler

The Barclays team found that over the past week, during which the S&P 500 hit a fresh record high, “skews across global indices collapsed as investors chased the rally and demand for protection faded.”

From MarketWatch ● Aug. 13, 2026

Huijin, the state-investment arm that acts as China’s plunge-protection team, stepped back into the market last week, buying broad-based exchange-traded funds tied to stock indices to stabilize the market.

From The Wall Street Journal ● Jul. 28, 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

That amount is indexed to increase with inflation starting next year, according to the IRS.

From MarketWatch ● Jul. 2, 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

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

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

“The problem with indexing is that a true market portfolio doesn’t exist,” Booth says.

From Barron's ● Jun. 18, 2026

If that’s too ambitious, they could at least raise or eliminate the cap on the capital-gains exemption on the sale of a primary residence, plus indexing it for inflation.

From The Wall Street Journal ● Jun. 11, 2026

Thanks to the visionary brilliance of Vanguard founder Jack Bogle and decades of strong returns, indexing is eating more and more of the market each year.

From Slate ● Jun. 5, 2026

By late 1966, Elizebeth and William were spending several hours a day indexing their library, at least when they were well enough.

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

Related Words

• indicator
• ratio