process ID

The unique identifier representing a process. A process ID is a positive integer. (Under ISO only, it is a positive integer that can be contained in a pid_t.) A process ID will not be reused by the system until the process lifetime ends. In addition, if there exists a process group whose process group ID is equal to that process ID, the process ID will not be reused by the system until the process group lifetime ends. A process that is not a system process will not have a process ID of 1. X/Open. ISO.1.