An index is a list of words or phrases and associated pointers 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.The definition of an index is a guide, list or sign, or a number used to measure change. An example of an index is a list of employee names, addresses and phone numbers. An example of an index is a stock market index which is based on a standard set at a particular time