Defensive rating or defensive efficiency is a statistic used in basketball to measure an individual player's efficiency at preventing the other team from scoring points. It was created by author and statistician Dean Oliver.
Formula
The formula is: Defensive Rating = (Opponent's Points Allowed/ Opponent's Possessions) x 100. The result is the expected amount of points that an individual player will allow on defense over 100 possessions. This stat can be significantly influenced by the defense of a player's teammates.
References
Defensive rating Wikipedia(Text) CC BY-SA