An alphabetical list of names, subjects, etc., with references to the places where they occur, typically found at the end of a book. The book's index helped me quickly find the chapter on climate change.
A sign or measure of something. The stock market index rose by 2% today.
A number or ratio derived from a series of observations and used as an indicator or measure. The consumer price index is used to measure inflation.
A pointer or indicator, especially on a measuring instrument. The index on the gauge showed that the pressure was too high.
A symbol or number used to denote a power or root in mathematics. In the expression x^2, the number 2 is the index.
(Computing) A data pointer or structure that enables fast retrieval of information within a larger set, such as a database table or an array. The database uses a B-tree index to speed up search queries.
(Anatomy, informal) The index finger. He raised his index to signal silence.