process ID

from The Free On-line Dictionary of Computing (8 July 2008)

process identifier
PID
process ID

   <operating system> (PID) An integer used by the {Unix}
   {kernel} to uniquely identify a process.  PIDs are returned by
   the {fork} {system call} and can be passed to wait() or
   kill() to perform actions on the given process.

   (1996-12-09)