I read "index" here to be a value computed to represent a variety of data and to represent something in a single value. This of a stock exchange index that is supposed to be a measure of the stock market in a single value.
It would be item two in this definition;
1. Something that serves to guide, point out, or otherwise facilitate reference, especially:
a. An alphabetized list of names, places, and subjects treated in a printed work, giving the page or pages on which each item is mentioned.
b. A thumb index.
c. A table, file, or catalog.
d. Computer Science A list of keywords associated with a record or document, used especially as an aid in searching for information.
2. Something that reveals or indicates; a sign: "Her face . . . was a fair index to her disposition" (Samuel Butler).
3. A character () used in printing to call attention to a particular paragraph or section. Also called fist, hand.
4. An indicator or pointer, as on a scientific instrument.
a. Mathematics A number or symbol, often written as a subscript or superscript to a mathematical expression, that indicates an operation to be performed, an ordering relation, or a use of the associated expression.
b. A number derived from a formula, used to characterize a set of data.
6. A number that represents the change in price or value of an aggregate of goods, services, wages, or other measurable quantity in comparison with a reference number for a previous period of time.
7. Index Roman Catholic Church A list formerly published by Church authority, restricting or forbidding the reading of certain books.