Indices

indices
(irregular plural of index)
(when used in the sense of a key to locate an element in a list, vector or matrix in computing or mathematics)The subscript numbers after each element are the indices of that element.A common convention in computing is to have indices beginning at zero, whereas in mathematics indices usually begin at one.

1972, American Society for Metals, Materials Science and Engineering, volumes 9â€“10, page 67 (Elsevier Sequoia)To locate any term, and to facilitate use of the dictionary by a national of any of the three languages, alphabetical indices are appended in English, French and German.
1976, Sidney Heath, The Romance of Symbolism and Its Relation to Church Ornament and Architecture, page 98 (Gale Research Co.; ISBN 0810343029, 9780810343023)Perhaps the feature which most contributes to making this a valuable reference book is the appendices and indices which are an integral part of the guide.
1989, TAPPI Journal, volume 72, page 204 (Technical Association of the Pulp and Paper Industry)Alphabetical indices of the terms in the other languages are given at the end of the book.
1999, Institute of Oriental Studies of the Saint-Petersburg Branch of the Russian Academy of Sciences, Manuscripta Orientalia: International Journal for Oriental Manuscript Research, volume 5, page 71 (THESA)The dictionaryâ€™s four indices are helpful when searching terms. These are: Hanyu Pinyin System Index; Four-Corner System Index; Radical Index; and Japanese Reading System Index.
2000, Russian Language Journal, volume 54, issues 177â€“179, page 295 (Michigan State University, Department of German & Russian)Its word indices makesÂ finding individual words easy.

Both plurals are in common use. The OED claims that when referring to a directory listing, the plural indexes should be used.