Plural of “index”

The plural of index is indices (some dictionaries also accept indexes).

Why

"Index" comes from Latin, where it originally meant "an indicator" or "a forefinger" (the pointing finger). Following the Latin third-declension pattern for words ending in -ex/-ix, its classical plural is "indices" — the same -ex/-ices pattern seen in "vertex/vertices" and "apex/apices."

"Indexes" — the fully regularized English plural — is now the dominant and often preferred form for the everyday sense of "index" (the alphabetical listing at the back of a book, or a stock market index), while "indices" survives more strongly in mathematical, statistical, and economic contexts (price indices, financial indices), a split by register rather than a strict right-or-wrong distinction. The verb "to index" (meaning to create a reference listing, or in finance, to link a value to a benchmark) is fully regular in every tense (indexes, indexed, indexing) regardless of which noun plural — "indices" or "indexes" — a given piece of writing prefers, since the verb form never participates in the Latin/English plural debate that only affects the noun.

Usage note

If you're writing about books, databases, or general reference material, "indexes" is the safer, more natural-sounding choice; if you're writing in a mathematical, scientific, or economics-adjacent register, "indices" will read as more standard and expected by that audience. The Latin plural "indices" is also the standard, essentially unchallenged term in computer programming for referring to multiple array or list positions ("loop over the indices"), a technical usage where "indexes" would actually sound slightly off to most working programmers, reversing the more general-writing preference for the regularized form.

In finance and economics writing specifically, "indices" remains strongly preferred even in publications that otherwise favor plain, non-technical language, likely because the word functions almost as industry jargon there, signaling technical authority in a way "indexes" doesn't carry in that specific context. The everyday "index finger" (the pointing finger) keeps the word's original literal Latin sense of "pointer" alive in ordinary anatomical vocabulary, and its plural is always the fully regular "index fingers" — never "indices" — since the anatomical sense of "index" never entered the Latin-plural debate that applies only to the reference-listing and mathematical senses of the word. Library science historically distinguished a "concordance" from an "index" by scope and method, though both share the same modern plural debate between "indexes" and the rarer "indices" in that specific reference-book context.

Common real mistakes

This is more a register mismatch than a genuine error — calling a book's back-of-book listing "indices" can sound needlessly formal, while a statistics paper reaching for "indexes" may strike specialists in that field as a touch too casual. Publishing itself adds a real ambiguity: a single book might contain several different indexes (a general index, a name index, a subject index), and in that specific context "indexes," not "indices," is the universal choice, even among academic presses that otherwise reach for the classical Latin plural in mathematical and statistical writing.

The wider pattern

The same Latin -ex/-ices pattern appears in "vertex/vertices," "apex/apices," "codex/codices," and "matrix/matrices" (matrix technically follows a related but distinct -ix/-ices pattern) — all Latin-derived technical or mathematical vocabulary that shows the same register-based split between the classical plural and a regularized English one.

Examples

• The book includes a detailed index of key terms.
• Several stock market indices fell sharply this week.
• The librarian updated the indexes for all the reference volumes.
• Economists track multiple price indices to measure inflation.
• Both indexes at the back of the book are alphabetized.
• The report compared indices across five different countries.
• The appendix includes a cross-referenced index.
• Analysts watched several key indices throughout the day.
• The publisher combined the indexes into one master list.
• Global indices dropped after the announcement.