1. in economics, the principle that 80% of the wealth in any society will be owned by 20% of the population.

2. more generally, the rule of thumb that 80% of any given output is produced by 20% of input. For example, in any retail organization, 80% of the sales will be accounted for by 20% of the customers. Also called 80:20 rule. [Vilfredo Pareto (1848–1923), Italian economist and sociologist]