The 80/20 Rule

20% of the inputs account for 80% of the outputs.

The Pareto principle (also known as the 80/20 rule, the law of the vital few, or the principle of factor sparsity) states that, for many events, roughly 80% of the effects come from 20% of the causes. Management consultant Joseph M. Juran suggested the principle and named it after Italian economist Vilfredo Pareto, who noted the 80/20 connection while at the University of Lausanne in 1896, as published in his first work, Cours d'économie politique. In it, Pareto showed that approximately 80% of the land in Italy was owned by 20% of the population.

It is an axiom of business management that "80% of sales come from 20% of clients".

Mathematically, the 80/20 rule is roughly followed by a power law distribution (also known as a Pareto distribution) for a particular set of parameters, and many natural phenomena have been shown empirically to exhibit such a distribution.

The Pareto principle is only tangentially related to Pareto efficiency. Pareto developed both concepts in the context of the distribution of income and wealth among the population.

Source
Vilfredo Pareto
https://en.wikipedia.org/wiki/Pareto_principle