in·dex | \ˈin-ˌdeks \
plural indexes or indices\ˈin-də-ˌsēz \

Definition of index 

(Entry 1 of 2)

1 : a list (as of bibliographical information or citations to a body of literature) arranged usually in alphabetical order of some specified datum (such as author, subject, or keyword): such as

a : a list of items (such as topics or names) treated in a printed work that gives for each item the page number where it may be found

b : a list of publicly traded companies and their stock prices

c : a bibliographical analysis of groups of publications that is usually published periodically

d : thumb index

2a : a number (such as a ratio) derived from a series of observations and used as an indicator or measure specifically : index number

b : the ratio of one dimension of a thing (such as an anatomical structure) to another dimension

3a : a device (such as the pointer on a scale or the gnomon of a sundial) that serves to indicate a value or quantity

b : something (such as a physical feature or a mode of expression) that leads one to a particular fact or conclusion : indication

4 plural usually indices : a number or symbol or expression (such as an exponent) associated with another to indicate a mathematical operation to be performed or to indicate use or position in an arrangement 3 is the index of the expression {latex}\sqrt[3]{5}{/latex} to indicate the cube root of 5

5 : a character ☞ used to direct attention to a note or paragraph

called also fist

6 : a list of restricted or prohibited material specifically, capitalized : a formerly published list of books the reading of which was prohibited or restricted for Roman Catholics by the church authorities


indexed; indexing; indexes

Definition of index (Entry 2 of 2)

transitive verb

1a : to provide with an index

b : to list in an index all persons and places mentioned are carefully indexed

2 : to serve as an index of

3 : to regulate (wages, prices, interest rates, etc.) by indexation

intransitive verb

: to index something

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Other Words from index


indexer noun

Synonyms for index

Synonyms: Noun

hand, indicator, needle, pointer

Synonyms: Verb

catalog (or catalogue), enroll (also enrol), enter, inscribe, list, put down, record, register, schedule, slate

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Examples of index in a Sentence


Look up the recipe for potato soup in the index. Potato soup is listed under “soup” in the index. The card catalog is an index to the materials in the library. the index on a scale


This search engine has indexed hundreds of millions of Web sites. indexed all the books in the library by category
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Recent Examples on the Web: Noun

An activist hedge fund index tracked by HFR Inc. returned 5.5% last year compared with the S&P 500’s 22% rise with dividends. Cara Lombardo, WSJ, "Activist Investors Turn Up Heat in Drive for Returns," 12 July 2018 An index of hedge fund performance, calculated by the research firm HFR, gained just 0.81 percent in the first half of 2018. James B. Stewart, New York Times, "Hedge Funds Should Be Thriving Right Now. They Aren’t.," 12 July 2018 The Dow is an index of the stock price for 30 companies from different sectors of the economy and is seen as one of the best gauges of market activity. James Pilcher,, "GE dropped from Dow, but locals not worried," 20 June 2018 At the back of the book, Hayes has inserted an index of first lines for each section. Walton Muyumba,, "The right poetry collection for right now: Terrance Hayes' 'American Sonnets for My Past and Future Assassin'," 14 June 2018 The Russell 2000, an index that makes up mostly small companies, rose 7.62 points, or 0.5 percent, to 1,682.30. Washington Post, "Asian shares drop as eyes turn to central bank meetings," 12 June 2018 Under a licence from NSE, punters could trade a derivative linked to the Nifty 50, an index which is to India what the FTSE 100 is to Britain or the S&P 500 is to America. The Economist, "Two Asian stock exchanges tussle over market data," 7 June 2018 An index of banking stocks fell 3.4 percent, the most in three weeks, while shares in general retailers dropped 2.3 percent. Colleen Goko,, "South African Assets Tumble as Ramaphosa's Growth Pledge Palls," 5 June 2018 The handicap requirements for gross is a GHIN Index of 9.0 or less, and net is an index of 18.0 or less. Drew Davison, star-telegram, "Trinity Forest to host inaugural Dallas amateur golf championships," 29 May 2018

Recent Examples on the Web: Verb

Of course, Google already has all of that information indexed, whether it’s puppy breeds, restaurant menus, clothing inventory, or foreign languages. Lauren Goode, WIRED, "Lens, Google's Visual Search Tool, Gets Its Most Significant Update Yet," 8 May 2018 At President Reagan’s behest, Congress in the 1980s indexed much of the federal tax code for inflation. Kimberley A. Strassel, WSJ, "Trump Alone Can Cut Taxes," 12 July 2018 The income thresholds are doubled for married couples and indexed to inflation. Kathleen Pender,, "Dozens of millionaires fled California after 2012 tax increase, study says," 6 July 2018 Congress initially made 90 percent of total income eligible for Social Security taxes, but indexed it to average wage growth. Ken Fisher, USA TODAY, "4 Social Security myths and the reasons why they're overblown," 24 June 2018 Each event is indexed so a particular play can later be viewed with a single tap. Tom Simonite, WIRED, "Apple's Plans to Bring Artificial Intelligence to Your Phone," 6 June 2018 European earnings have increased since then, but the U.S. has grown much faster—with the gap in earnings per share between MSCI’s USA and Europe indexes ballooning to a 30-year high, according to Mislav Matejka, equity strategist at JPMorgan. Riva Gold, WSJ, "America’s Profit Boom Leaves Europe Inc. Playing Catch-Up," 23 May 2018 Emails and other documents show that Washington County has a database of more than 300,000 mugshots that is indexed by Rekognition. Dan Goodin, Ars Technica, "Police use of Amazon’s face-recognition service draws privacy warnings," 22 May 2018 The Orange County district is no longer looking to use a rate benchmark indexed by the Metropolitan Water District. Priscella Vega,, "O.C. Water District delays vote on Huntington Beach desalination project’s rate plan," 7 June 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'index.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of index


1561, in the meaning defined at sense 3a


1720, in the meaning defined at transitive sense 1a

History and Etymology for index


Latin indic-, index, from indicare to indicate

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Statistics for index

Last Updated

15 Oct 2018

Look-up Popularity

Time Traveler for index

The first known use of index was in 1561

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More Definitions for index



Financial Definition of index

What It Is

An index is a statistical aggregate that measures change. In finance, they usually refer to measures of stock market performance or economic performance.

How It Works

Let's say we want to measure the stock price performance of the widget industry. There are currently four public companies that make widgets in the United States: Company A, Company B, Company C, and Company D. In the year 2000, when we started caring about the seedling U.S. widget industry, the four companies' stock prices were as follows:

Company A     $10
Company B     $8
Company C     $12
Company D     $25

Total $55

To create an index, we simply set the total ($55) in the year 2000 equal to 100 and measure any future periods against that total. For example, let's assume that in 2001 the stock prices were:

Company A     $4
Company B     $38
Company C     $12
Company D     $24

Total $78

Because $78 is 41.82% higher than the 2000 base, the index is now at 141.82. Every day, month, year, or other period, the index can be recalculated based on current stock prices.

Note that this index is weighted by stock price (i.e., the larger the stock price, the more influence it has on the index). Indexes can be weighted by shares outstanding, market capitalization, or any other factors the indexer chooses. When new companies go public or existing companies founder, the indexer may add or delete companies from the index or "reweight" the index to accommodate stock splits or other factors.

Why It Matters

In finance, the most significant numbers in any given day's news are usually market indices. The Dow Jones Industrial Average is probably the best-known and most widely followed financial index in the world. It consists of 30 The definition of index on InvestingAnswersof the largest publicly traded firms in the United States. The S&P 500 Index is also very common, comprising over 70% of the total market cap of all stocks traded in the U.S. The Nasdaq Composite is a broad market index that encompasses about 4,000 issues traded on the Nasdaq National Market -- virtually every firm that trades on the exchange.

Indices are also used to gauge activity in an economy. Perhaps the best known economic index in the United States is the CPI, or Consumer Price Index, which measures inflation.

Source: Investing Answers



English Language Learners Definition of index

: to provide an index for (something, such as a book)

: to list or include (something) in an index

: to link wages, benefits, etc., to a measurement of changes in the price of goods and services so that they increase at the same rate


in·dex | \ˈin-ˌdeks \
plural indexes or indices\ˈin-də-ˌsēz \

Kids Definition of index

 (Entry 1 of 2)

1 : a list of names or topics (as in a book) given in alphabetical order and showing where each is to be found

2 : pointer sense 1 the index on a scale

3 : sign entry 1 sense 3, indication Prices are an index of business conditions.


indexed; indexing

Kids Definition of index (Entry 2 of 2)

1 : to provide (as a book) with an index

2 : to list in an index The topics are indexed.

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in·dex | \ˈin-ˌdeks \
plural indexes or indices\-də-ˌsēz \

Medical Definition of index 

2 : a list (as of bibliographical information or citations to a body of literature) arranged usually in alphabetical order of some specified datum (as author, subject, or keyword) Index Medicus of the United States National Library of Medicine

3a : a ratio or other number derived from a series of observations and used as an indicator or measure (as of a condition, property, or phenomenon) physiochemical indexes of the urine, the blood, and the gastric juiceJournal of the American Medical Association

b : the ratio of one dimension of a thing (as an anatomical structure) to another dimension — see cephalic index, cranial index



Legal Definition of index 

(Entry 1 of 2)

: a numerical measure or indicator (as of inflation or economic performance) — see also consumer price index

Legal Definition of index (Entry 2 of 2)

: to link (as wages, rates, or investments) to an index under the contract wages were indexed to inflation

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