The Gini Index is a measure of a country's income inequality.
Corrado Gini developed the Gini Coefficient in his 1912 paper "Variability and Mutability" to express a distribution's difference from uniformity. The Gini Index, based on Gini's work, is used to describe the income inequality in a population, commonly between countries and within countries.
Countries publish their own Gini Index. Many institutions, including the World Bank and the CIA calculate the Gini Index of countries world-wide.
Income Inequality within 'optimal limits' promotes growth. The rate of income inequality in the world's wealthy countries avoids extreme egalitarianism and extreme inequality. There is no correlation between Gini and wealth for poorer countries.