A running instance of a proggram is called a process.
Process identifier (normally referred to as the process ID or just PID) is a number to temporarily uniquely identify a process. This number may be used as a parameter in various function calls allowing processes to be manipulated, such as adjusting the process's priority or killing it altogether.
In Unix-like operating systems, new processes are created by the fork() system call. The PID is returned to the parent enabling it to refer to the child in further function calls. The parent may, for example, wait for the child to terminate with the waitpid()...