A figure used to show the variation in some quantity over a period of time, usually standardized relative to some base value. The index is given as a percentage with the base value equal to 100%. For example, the retail price index is used to measure changes in the cost of household items. Indices are often calculated by using a weighted mean of a number of constituent parts.
http://www.netcomuk.co.uk/~jenolive/sindexes.html Examples of calculations of index numbers.