The Gini index refers to a simple measure of the distribution of income across income percentiles in a population.

Greater inequality is represented by higher Gini index, i.e. individuals with high income are receiving much larger percentages of the total income of the population.

Any country with every resident receiving the same income would have an income Gini coefficient of 0. On the other hand, any country in which one resident earned all the income, while everyone else earned nothing, would have an income Gini coefficient of 1.

The same analysis can be applied to wealth distribution (the "wealth Gini coefficient"), but because wealth is more difficult to measure than income. Gini coefficients usually refer to income and appear simply as "Gini coefficient" or "Gini index," without specifying that they refer to income. Wealth Gini coefficients tend to be much higher than those for income.

A high-income country and a low-income one can have the same Gini coefficient, as long as incomes are distributed similarly within each.

The Gini index is often represented graphically through the Lorenz curve, which shows income (or wealth) distribution by plotting the population percentile by income on the horizontal axis and cumulative income on the vertical axis.