A P0 (P-zero) also known as a priority zero defect is a classification convention used for critical issues to define the order of prioritization or resolution of the defect. Priorities are discussed and decided on the bug triaging meetings between the team during the project execution and P0 defects assume the highest priority as they signify the most critical features/areas of the application under test that need to be fixed with the organization-defined service level agreements (SLA's). In general, below is the priority classification that is followed:  P0 - Critical  P1 - High  P2 - Medium  P3 - Low/Cosmetic  It is important to note that this classification is also applicable to test cases in the same way as defects, to determine the order of execution of test cases.