Isolated Power or ISO is a sabermetrics baseball statistic which aims to measure a batter's raw power. The formula is slugging percentage minus batting average. The final result measures how many extra bases a player averages per at bat. A player who hits only singles would thus have an ISO of 0. The maximum ISO is 3.000, and could only be attained by hitting a home run in every at-bat. Power hitters ordinarily have an ISO between .240 and .300.
The term "Isolated Power" was coined by Bill James, but the concept dates back to Branch Rickey and his statistician Allan Roth. There are uncertain memories that it was called "power percentage" by Rickey/Roth, and while rare, that has led to that name being used in some circles over the past 35 years.