An index (PL: usually indexes, more rarely indices; see below) is a list of words or phrases (´headings´) and associated pointers (´locators´) to where useful material relating to that heading can be found in a document or collection of documents. Examples are an index in the back matter of a book and an index that serves as a library catalog. An index differs from a word index, or concordance, in focusing on the subject of the text rather than the exact words in a text, and it differs from a table of contents because the index is ordered by subject, regardless of whether it is early or late in the book, while the listed items in a table of contents is placed in the same order as the book.
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