Market Index

The market index is a measurement of the performance of a specific "basket" of stocks considered to represent a particular market or sector of the U. S. economy. For example, the Dow Jones Industrial Average (DJIA) is an index of 30 "blue chip" stocks of U.S. companies.

Source: http://investor.gov/glossary/glossary_terms/market-index#.Us7rRWRDuEg