Real Dictionary

index
countable noun: An index is a system by which changes in the value of something and the rate at which it changes can be recorded, measured, or interpreted.

index in American English
index finger
noun: a more or less detailed alphabetical listing of names, places, and topics along with the numbers of the pages on which they are mentioned or discussed, usually included in or constituting the back matter

index in British English
to put an index in (a book)
noun: an alphabetical list of persons, places, subjects, etc, mentioned in the text of a printed work, usually at the back, and indicating where in the work they are referred to

index in Finance
noun: An index is a grouping of the combined values of stocks or securities that is used to track changes over time.

index in Insurance
noun: An index is a system by which prices and costs can be compared to those of a previous date.

index in Mechanical Engineering
verb: To index a machine or a workpiece held in a machine tool means to move it so that one particular operation will be repeated at particular intervals.