price indexn. A number relating prices of a group of commodities to their prices during an arbitrarily chosen base period.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.price? in`dex n. an index of the changes in the prices of goods and services, based on the prices of a previous period, with the base level usu. expressed as 100. [1885?90] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.