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?
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.
For American English we have (in both charts, indices is blue and indexes is red):
For British English we have:
OED states:
Forms: Pl. indexes (also 16 index's) and indices /ˈɪndɪsiːz/ .
Etymology: < Latin inde, indic-em, plural indicēs, the forefinger, an informer, sign, inscription. [...]
In current use the plural is indices in senses 8, 9, and usually in other senses except 5, in which indexes is usual.
And the respective senses are: (5): index in the end of the book etc.; (8): mathematical index, and (9):
9. In various sciences, a number or formula expressing some property, form, ratio, etc. of the thing in question.
As others have said, either one is correct.
Unfortunately, the problem goes far beyond this particular noun to pretty much all nouns of direct Latin descent. As people study Latin less and less, the use of Latin plural forms is slowly eroding. In general, the more a word has found its way into common usage, the more likely that the English plural has replaced the Latin.
For example, "status" is both the singular and plural form in Latin, but "statuses" is common even though it sounds awkward, presumably because we want to have different words for singular and plural. 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," Similarly, one item of data is a datum, but we rarely say "the data are on the way," and the word "datum" is very rare; people usually prefer to say something like "a unit of data." And I don't see "fora" in place of "forums" except in forums that I would never join. :)
On the other hand, we generally say "appendices," I suspect primarily because "appendixes" sounds funny. I see both "curricula" and "curriculums." Further, though, I never see "genuses," always "genera," and likewise for "quantums" vs. "quanta."
Then, "news medium" and "news media" are always used, while "mediums" are people who hold séances and call up ghosts. So, what we do is rather arbitrary.
Both indexes and indices are correct. A question like this can be settled by checking a good dictionary. I checked my answer by going to OneLook.com, a great starting point for many online English dictionaries.
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.
