Forum: ell.stackexchange.com
Tags: <plural-forms><word-form><latin>
Title: Is "indices" or "indexes" the plural of "index"?
Question: I've heard both plural forms of  index ,  indices  and  indexes . I usually use  indices  when referring to the computer science term for  database index , but I'm not sure if it is correct in that context.   Are both  indices  and  indexes  correct? If  only one of them is correct, which one?

Answers:
sents: text: Both are correct , though there are some specific usages as pointed in another answer; Google Ngrams suggests that indices is slightly more used than indexes .
text: For American English
text: we have (in both charts, indices is blue and indexes is red):
text: For British English
text: we have:
answer_details: author: ell.stackexchange.com/users/24/Renan
score: 20
sents: text: OED states : Forms : Pl. indexes (also 16 index's ) and indices /ˈɪndɪsiːz/ .
text: Etymology : < Latin inde , indic-em , plural indicēs , the forefinger, an informer, sign, inscription.
text: [...]
text: In current use the plural is indices in senses 8, 9, and usually in other senses except 5, in which indexes is usual.
text: And the respective senses are: (5):
text: index in the end of the book etc. ; (8): mathematical index, and (9): 9.
text: In various sciences, a number or formula expressing some property, form, ratio, etc.
text: of the thing in question.
answer_details: author: ell.stackexchange.com/users/56/theUg
score: 8
sents: text: Both indexes and indices are correct .
text: A question like this can be settled by checking a good dictionary.
text: I checked my answer by going to OneLook.com , a great starting point for many online English dictionaries.
answer_details: author: ell.stackexchange.com/users/41/MetaEd
score: 5
sents: text: Unfortunately, the problem goes far beyond this particular noun to pretty much all nouns of direct Latin descent.
text: As people study Latin less and less, the use of Latin plural forms is slowly eroding.
text: In general, the more a word has found its way into common usage, the more likely that the English plural has replaced the Latin.
text: For example, "status" is both the singular and plural form in Latin, but "statuses" is common even though it sounds awkward.
text: We all speak of "an agenda", when actually "some agenda" is correct in the Latin sense; one of a group of agenda is an "agendum".
text: Similarly, one item of data is a datum, but we rarely say "the data are on the way."
text: I don't see "fora" in place of "forums" except in forums that I would never join. :)
text: On the other hand, we generally say "appendices", I suspect primarily because "appendixes" sounds funny.
text: I see both "curricula" and "curriculums".
text: Further, though, I never see "genuses", always "genera", and likewise for "quantums" vs. "quanta".
answer_details: author: ell.stackexchange.com/users/1883/BobRodes
score: 5
sents: text: Both are correct, though indexes is more commonly associated with the index reference of a book, while indices tends to be used with numerical symbols and ratios.
answer_details: author: ell.stackexchange.com/users/107/Walrus
score: 3

Summary of the answers:
Both  indexes  and  indices  are  correct  although  indexes  is more commonly associated with the index reference of a book, while  indices  tends to be used with numerical symbols and ratios. OED  states:   Forms:  Pl.  indexes  (also 16  index's ) and  indices  /ˈɪndɪsiːz/ .
Indexes is used more in referencing, indices in numerical symbols and rations. Both are correct.