1. Normally an alphabetical listing of subjects, of people, of places, or of topics; giving the numbers of the pages where they are located; the purpose of which is to assist the reader in finding specific information: "As a graduate assistant to the linguistics and language professor, Carrie created the indexes for Dr. John's two new books about various sources of current vocabulary words."
2. Something which is used to point out, to identify information on a page or a projected screen, etc.: "Jason used an index which had a bright light at the end of it to indicate the important information on his power point projection."
3. Etymology: "the forefinger (used for pointing)", from Latin index, indicis. "forefinger, pointer, sign, list, catalogue"; literally, "anything that points out", from indicare, "to point out, to indicate".

The meaning of a "list of the contents of book's" is first recorded in 1580, from Latin phrases; such as, Index Nominum, "Index of Names", index expurgatorius, "specification of passages to be deleted from works otherwise permitted"; and Index Verborum, "Index of Words" which was used as headings in English books of that time.

The Catholic Church sense of "forbidden books" is from index librorum prohibitorum, which was first published in 1564 by the authority of Pius IV.

To put information in an alphabetical or logical sequence for the use of a reader: "For his summer job experience, Leon was indexing the information about community service organizations for his city councilor."

A measure of the relative content of the antibody opsonin in the blood of a person with an infectious disease, as compared to te opsonin content in normal blood.