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Wiktionary Word definitions in Wiktionary
n. (en-irregular index nodot=1) (gloss: when used in the sense of a key to locate an element in a list, vector or matrix in computing or mathematics)

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Index \In"dex\, n.; pl. E. Indexes , L. Indices (?). [L.: cf. F. index. See Indicate , Diction .] That which points out; that which shows, indicates, manifests, or discloses; as, the increasing unemployment rate is an index of how much the economy has slowed. ...

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
according to OED, the plural form of index preferable in scientific and mathematical senses of that word.

WordNet Word definitions in WordNet
See index