Gini coefficient is a measure of inequality. Gini coefficient is based on Lorenz's curve, and the range of this coefficient is between 0 and 1. The more lower the value, the more equality the distribution is. The algorithm of this function is directly adopted from package "ineq" by Achim Zeileis.
References
F A Cowell: Measurement of Inequality, 2000, in A B Atkinson / F Bourguignon (Eds): Handbook of Income Distribution, Amsterdam