Team isolated power takes the number of total bases a team accumulated over the course of the season, subtracts from that the number of singles the team hit, and divides that difference by the total number of team games played.
ISO = ((2B) + (2*3B) + (3*HR)) / AB
More about Team Isolated Power
This is similar to team slugging percentage, but differs in that total bases accumulated from singles are not included in the figure, meaning that teams that hit more singles than the average team as part of all of their hits will have a lower isolated power, which is intended to measure power as determined by team extra base hits.