What is the 80/20 rule?

The 80/20 rule can be applied in many contexts. Casually, it may refer to the observation that often 20% of the effort yields 80% of the benefit and the corollary that the last 20% of the benefit requires 80% of the effort. The 80/20 rule also describes a property that is empirically true of the frequency distribution of many sets, such as 20% of unique words accounting for 80% of words in a book or 20% of companies in a stock portfolio accounting for 80% of the return.