index number

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A number indicating change in magnitude, as of price, wage, employment, or production shifts, relative to the magnitude at a specified point usually taken as 100.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time