The Gini coefficient is a number between 0 and 1, where 0 means perfect equality (everyone has the same income) and 1 means perfect inequality (one person has all the income, everyone else earns nothing).
While the Gini coefficient is mostly used to measure income inequality, it can be used to measure wealth inequality as well.
The Gini coefficient is calculated using areas on the Lorenz curve diagram. If the area between the line of perfect equality and Lorenz curve is A, and the area underneath the Lorenz curve is B, the Gini coefficient is A/(A+B). This is often expressed as a percentage.