population pyramid n (Sociology) a pyramid-shaped diagram illustrating the age distribution of a population: the youngest are represented by a rectangle at the base, the oldest by one at the apex Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014population pyramidA diagram showing successive age groups as a pyramid of horizontal bars. Each bar?s length shows the relative size of one age group.Dictionary of Unfamiliar Words by Diagram Group Copyright © 2008 by Diagram Visual Information Limited