Index

An index is a structured list that facilitates retrieval of information. It can refer to a tool for locating material within a document or a broader measure used across disciplines to summarize or point to data.

In publishing, an index is an alphabetical compilation of topics, names, and places with page references, usually located at the end of a book. It helps readers locate material without scanning the entire text and often includes cross-references and subentries for related topics.

In information technology, an index is a data structure that improves search performance by mapping key values to their locations in a dataset. Common forms include B-tree and hash indexes; indexes incur storage and maintenance costs and must be updated as data changes. Indexing is a foundational technique in databases, file systems, and search engines.

In statistics and finance, a stock index aggregates the performance of a defined set of assets to reflect market trends. Indices provide benchmarks for investors and are used to construct index funds and other financial instruments.

In mathematics, the term index appears in several contexts: the index of a subgroup [G:H] measures the number of cosets; the dimension of a vector space is sometimes referred to as its index; and index notation is used for exponents or subscripts in formulas.

In linguistics and semiotics, an index is a sign that points to its referent through a causal or contextual link, such as smoke as an index of fire. Across disciplines, the common theme is that an index serves as a pointer, reference, or summary that facilitates access to information or measurement.