The 80-20 Rule
80% of outcomes result from 20% of causes.
While not as well-known as the bell-shaped Normal (Gaussian) distribution, the Pareto distribution is a powerful tool for modeling a variety of real-life phenomena. It is named after the Italian economist Vilfredo Pareto (1848-1923), who developed the distribution in the 1890s as a way to describe the allocation of wealth in society. He famously observed that 80% of society’s wealth was controlled by 20% of its population, a concept now known as the “Pareto Principle” or the “80-20 Rule”. Source.
Also see Pareto efficiency and Parkinson’s Law.