Gini Index
It's an instrument used to measure the degree of income concentration in a given group—in other words, to assess social inequality in a given region or country.
It indicates the difference between the incomes of the poorest and richest. Numerically, it ranges from zero to one (some show zero to one hundred). A value of zero represents equality, meaning everyone has the same income. A value of one (or one hundred) is the opposite extreme, meaning a single person holds all the wealth.