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An index (plural. indices) is a pointer that takes you to information. Traditionally indexes (also pluralised particularly in Europe as indices) are found at the back of books. They complement the contents page because they give access to information by its specific name; contents pages, on the other hand, give access through broad categories. Indexes are usually organised alphabetically; contents pages are organised according to the order of pages in the book.

Indexing skills have also been applied to periodicals and databases, and web indexing is now becoming important.

In mathematics, an index (plural indices) is a superscript[?] or subscript[?] to a symbol. Superscript indices are often, but not always, used to indicate powers. Subscript indices are usually used to label a set or sequence of variables.

The index of a subgroup is the number of its left cosets (which is equal to the number of its right cosets).

In computer science, an index is usually used for expressing an nth element. Indices are usually expressed as integers.

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