index

1 of 2

noun

in·​dex ˈin-ˌdeks 

plural indexes or indices ˈin-də-ˌsēz 

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

2

a

: 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

3

a

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

index

2 of 2

verb

indexed; indexing; indexes

transitive verb

1

a

: 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

indexer noun

Synonyms

Noun

• indicator
• needle

Verb

• list
• record
• enter
• catalog
• file

Examples of index in a Sentence

Noun

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

Verb

This search engine has indexed hundreds of millions of websites.

indexed all the books in the library by category

Recent Examples on the Web

Noun

Wall Street cheered the data, with major indexes climbing to record highs. —Benzinga, Freep.com, 25 Oct. 2025

The blue-chip index closed above 47,000 for the first time ever. —John Towfighi, CNN Money, 24 Oct. 2025

Verb

OpenAI’s trajectory mirrors the rise of Google parent Alphabet, which built its business around indexing the web and now has a foothold in everything from consumer tech devices to health research. —Lisa Eadicicco, CNN Money, 7 Oct. 2025

Social security is also indexed to inflation. —Aliss Higham, MSNBC Newsweek, 28 Sep. 2025

Word History

Etymology

Noun

Latin indic-, index, from indicare to indicate

First Known Use

Noun

1561, in the meaning defined at sense 3a

Verb

1720, in the meaning defined at transitive sense 1a

Phrases Containing index

• body mass index
• card index
• cephalic index
• consumer price index
• cost-of-living index
• cranial index
• cross-index
• facial index
• glycemic index
• heat index
• index card
• index case
• index finger
• index fossil
• index-linked
• index number
• index of refraction
• index patient
• misery index
• price index
• refractive index
• therapeutic index
• the retail price index
• thumb index
• viscosity index

Video

The True Story of the Backward Index

These people needed a computer

Kids Definition

index

1 of 2 noun

in·​dex ˈin-ˌdeks 

plural indexes

-ˌdek-səz

or indices -də-ˌsēz 

1

a

: a device (as the pointer on a scale) used to indicate a value or quantity

b

: something that leads a person to a particular fact or conclusion : indication

the price of goods is an index of the economy

2

: an alphabetical list in a printed work that gives with each item listed the page number where it may be found

3

plural usually indices : a mathematical figure, letter, or expression (as the exponent 3 in a³) showing a power or root of another

4

: a symbol ☞ used to direct attention

index

2 of 2 verb

1

a

: to provide with an index

index a book

b

: to list in an index

2

: to serve as an index of

indexer noun

Medical Definition

index

noun

in·​dex ˈin-ˌdeks 

plural indexes or indices -də-ˌsēz 

1

: index finger

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

3

a

: 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 juice

—The Journal 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

index

1 of 2 noun

in·​dex

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

index

2 of 2 transitive verb

: to link (as wages, rates, or investments) to an index

under the contract wages were indexed to inflation