An indicator of the degree of inequality is the Gini coefficient. It is a single number that can range between 0 (a perfectly equal distribution) and 1 (a perfectly unequal distribution). The historical trend in the United States is one of almost steadily increasing income inequality (from 0.386 in 1968 to 0.477 in 2011).
The U.S. Income Distribution and Mobility: Trends and International Comparisons
by Linda Levine, Specialist in Labor Economics
Congressional Research Service
CRS Report for Congress
Prepared for Members and Committees of Congress
http://www.fas.org/sgp/crs/misc/R42400.pdf