In this letter, we present a novel and important performance metric for analyzing and evaluating the physical layer security of wireless communication systems in the presence of eavesdropping users. Specifically, we introduce the concept of amount of secrecy loss and derive the expressions that can quantify the severity of loss in systems with eavesdropping users. In addition, a new method for calculating the average secrecy rate is proposed. We further evaluate the performance of cooperative communication systems with the proposed performance metric to observe the behavior of different relaying schemes. Finally, simulation results are provided to verify the accuracy of the presented analytical expressions.