Definition: 80/20 rule

A widely felt conviction that there is an inherent 80/20 relationship between cause and effect. Also called the "Pareto principle" and the "law of the vital few." For example, in a software system, 80% of the errors are caused by 20% of the code. Other examples are 20% of the features in a product are used 80% of the time, and 80% of revenue comes from 20% of the customers.