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 fire severity index —how severe a fire could be, should one to start — will come down to low or moderate, according to forecasts from Natural England and the Met Office.

From BBC ● Aug. 17, 2026

Dutch pharmaceutical company Argenx was leading the index risers—up around 7.7%—after reporting positive trial results.

From The Wall Street Journal ● Aug. 17, 2026

The ISM reported that 20% of the manufacturing GDP last month had an index reading that signaled contraction, up from 5% in June.

From The Wall Street Journal ● Aug. 15, 2026

Such readings are a sign of extreme dispersion beneath the surface of the index — and the fear is this may soon boil over.

From MarketWatch ● Aug. 15, 2026

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

From "Breaking Through" by Francisco Jiménez

Broad approach: An active fund holding a whopping 800 stocks is beating major indexes.

From MarketWatch ● Aug. 14, 2026

Tech-heavy indexes like Japan's Nikkei 225 seem to be moving in tandem with the Kospi's wild swings, Societe Generale's Benzimra says.

From BBC ● Aug. 13, 2026

The gains came even as major indexes posted modest moves in the wake of an as-expected report on consumer prices.

From The Wall Street Journal ● Aug. 12, 2026

July's price data offered some respite for households, with grocery prices dropping 0.1 percent over the month, with three of the six major grocery food group indexes showing declines.

From Barron's ● Aug. 12, 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

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

While this enthusiasm repeatedly pushed indices to unprecedented highs, the sustained 90-day rally left stock prices looking incredibly inflated.

From BBC ● Jun. 23, 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

"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

Vanguard will manage the indexed equity and indexed and active fixed-income sleeves in the new all-markets fund.

From The Wall Street Journal ● Jul. 22, 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

The remaining papers had to be “grouped, described, labeled, indexed, and reverently dispatched to their sealed tombs in the government vaults.”

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

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

Lastly, investors should want to avoid the implementation costs associated with the indexing approach that many equal-weighted and quality-focused strategies tend to use.

From MarketWatch ● Jun. 23, 2026

Booth started up DFA in his Brooklyn Heights, N.Y., brownstone in 1981, after forays in indexing at Wells Fargo and A.G.

From Barron's ● Jun. 18, 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