index

index — noun

1. an alphabetically ordered section in the final pages of a publication that shows

1.nounA2
Definition

an alphabetically ordered section in the final pages of a publication that shows the page numbers for topics, names, and subjects discussed throughout the work.

Example

Mark checked the index at the back of the textbook for the page on verbs.

noun + at the back of [a book]

Eve found the entry for 'climate change' in the index of the library book.

Common collocations
  • book index
  • subject index
  • author index
  • consult the index
  • check the index
Synonyms
  • list

    broader term; not necessarily alphabetical or page-numbered

  • catalogue

    often implies a systematic collection of items, not specific to books

  • directory

    usually lists contact information or locations, not page numbers

Usage Note

Not the same as a table of contents, which lists chapters in order at the front of a book. An index is alphabetical and appears at the back.

Common Mistakes

I looked at the index on the first page to find Chapter 3.
I looked at the table of contents on the first page to find Chapter 3.
The table of contents is at the front; the index is at the back.

2. a structured set of records kept in a computer filing system or on physical card

2.nounB1
Definition

a structured set of records kept in a computer filing system or on physical cards, designed to make retrieval fast.

Example

Ilan searched the computer's file index to find the document from last Tuesday.

computer's file index

Rafael built an index of all the patient records on the hospital database.

Common collocations
  • database index
  • card index
  • file index
  • search index
  • build an index
  • update an index
Synonyms
  • database

    contains the actual data, while an index points to where the data is

  • filing system

    less technical; any organized method of storing records

  • catalogue

    a systematic list of items, often with descriptive details

Usage Note

In computing, an index is a data structure that speeds up searches in a database. In libraries, a physical card index serves a similar purpose.

Common Mistakes

I saved the file in the computer index.
I saved the file in the computer database.
A database contains the actual data; an index helps locate it.

3. a set of numbers that shows how the value of a group of items, such as prices or

3.nounB2
Definition

a set of numbers that shows how the value of a group of items, such as prices or stocks, changes over time and allows comparisons against a standard.

Example

Marco watched the stock market index rise by three percent in one week.

stock market index rises / falls

The consumer price index showed that food costs had gone up by nearly five percent.

Common collocations
  • stock market index
  • price index
  • consumer price index
  • index rises
  • index falls
  • index of economic activity
Synonyms
  • benchmark

    a standard against which things are compared, often in performance

  • gauge

    a measurement tool; less formal than index in economic contexts

  • scale

    a range of values; less specific about the comparative function

Usage Note

Often used in compound noun phrases: 'stock market index', 'consumer price index', 'body mass index'.

Common Mistakes

The index of my salary is too low.
The index shows that salaries have not kept up with inflation.
An index measures a group, not an individual value.

4. a sign or piece of evidence that reveals the strength of a particular condition,

4.nounB2
Definition

a sign or piece of evidence that reveals the strength of a particular condition, feeling, or quality, providing a clue about a broader situation.

Example

Low attendance at the meeting was an index of how tired the team felt.

be an index of [something]

A child's willingness to share can be an index of their social development.

Common collocations
  • index of
  • be an index of
  • reliable index
  • useful index
  • index of economic health
Synonyms
  • indicator

    nearly interchangeable; more common in everyday use

  • sign

    simpler word; less formal, does not imply a systematic measure

  • measure

    focuses on the quantifiable aspect of the sign

Usage Note

Distinguish from noun sense 3 (COMPARISON SCALE): this sense points to a single observable sign, not a calculated numeric system. Common in academic writing: 'be an index of…'.

Common Mistakes

The test score is an index for how smart you are.
A test score is only one index of your knowledge in a subject.
'Index of' suggests a partial indicator, not a complete measure.

5. a numeral placed slightly above another numeral to show the count of times the l

5.nounB2
Definition

a numeral placed slightly above another numeral to show the count of times the lower number is used as a factor in multiplication.

Example

In 4³, the small number 3 is the index telling you to multiply 4 three times.

the index in [expression] tells you to multiply [base] [exponent] times

The teacher asked the class to find the index in two to the fifth power.

Common collocations
  • small index
  • large index
  • base and index
  • index notation
  • raised index
Synonyms
  • exponent

    more common term in everyday math for the raised number

  • power

    often used for the whole operation (e.g., '4 to the power of 3')

  • superscript

    describes the position of the number, not its mathematical function

Usage Note

In mathematics, 'index' is used interchangeably with 'exponent' or 'power'. The plural is 'indices' (/'ɪndɪsiːz/) in technical contexts, though 'indexes' is also accepted.

index — verb

Word family

  • indexation
    nounC1
  • indexer
    nounC1
  • indexed
    adjectiveB2
  • indices
    nounC1

Synonym discussion

Sense: noun/1

All four words refer to parts of a book that help readers find information, but they differ in location and purpose. A table of contents appears at the front of a book and lists chapters or sections in the order they appear — it gives you the big picture of the book's structure. A glossary sits at the back and defines specialised terms used in the book; it is not organised by page number. An appendix contains extra material such as charts, tables, or documents that support the main text. An index, also at the back, is an alphabetical list of topics, names, and subjects with their page numbers — it helps you locate specific information quickly. For example, you use the table of contents to find Chapter 5, but you use the index to find every page that mentions 'Napoleon'.

index ↗table of contentsglossary ↗appendix ↗

Common collocations

Nouns

Adverbs

Phrases

  • at the back of the book
  • by subject
  • in alphabetical order
  • to inflation

Prepositions

Etymology

LatinEnglish

Index comes from Latin 'index', meaning 'forefinger' or 'pointer', from the verb 'indicare' (to point out), which combines 'in-' (toward) and 'dicere' (to say or declare). In Latin, the forefinger was seen as the finger used for pointing. The word entered English in the 14th century with the general meaning of 'something that points out or indicates'. The specific book-reference meaning emerged in the late 1500s, when printed books began including alphabetical lists of topics at the end. The mathematical sense — a superscript indicating how many times to multiply — dates from the 17th century. The economic and financial uses developed in the 19th and 20th centuries.