Quiz: Remember the definition of mal de mer?
according to OED, the plural form of index preferable in scientific and mathematical senses of that word.
late 14c., "the forefinger," from Latin index (genitive indicis) "forefinger, pointer, sign, list," literally "anything which points out," from indicare "point out" (see indication). Meaning "list of a book's contents" is first attested 1570s, from Latin phrases such as Index Nominum "Index of Names," index expurgatorius "specification of passages to be deleted from works otherwise permitted." Scientific sense (refractive index, etc.) is from 1829; economic sense (cost-of-living index, etc.) is from 1870, from the scientific usage, from sense "an indicator." The Church sense of "forbidden books" is from index librorum prohibitorum, first published 1564 by authority of Pius IV.
"compile an index," 1720, from index (n.). Related: Indexed; indexing.
index in·dex (ĭn'děks')
n. pl. in·dex·es or in·di·ces (-dĭ-sēz')
A guide, standard, indicator, symbol, or number indicating the relation of one part or thing to another in respect to size, capacity, or function.
A core or mold used to record or maintain the relative position of a tooth or teeth to one another or to a cast.
A guide, usually made of plaster, used to reposition teeth, casts, or parts.
The index finger.
An alphabetical list of subjects treated in a book. It usually appears at the end of the book and identifies page numbers on which information about each subject appears.
A plural of "index".