Market Index

An aggregate value produced by combining several stocks or other investment vehicles together and expressing their total value against a base value from a specific date. Market indexes are intended to represent an entire stock market and thus track the market's changes over time.

Investopedia Says:
Index values are useful for investors to track changes in market values over long periods of time. For example, the widely used Standard and Poor's 500 Index is computed by combining 500 large-cap U.S. stocks together into one index value. Investors can track changes in the index's value over time and use it as a benchmark to compare their own portfolio returns to.

Related Links:
Get to know the most important market indices and the pros and cons of investing in them. Index Investing
Be in the know - learn about the five most talked about indexes and what makes them all different. A Market By Any Other Name
Learn how a change in leadership could mean more taxes for you. Will A New Fund Manager Cost You?

Numbers representing weighted values of the components that make up the index. A stock market index, for example, is weighted according to the prices and number of outstanding shares of the various stocks. The Standard & Poor's 500 Stock Index is one of the most widely followed, but myriad other indexes track stocks in various industry groups.