Source : Free On-Line Dictionary of Computing

pre-emptive multitasking
     
         A type of {multitasking} where
        the {scheduler} can interrupt and suspend ("swap out") the
        currently running task in order to start or continue running
        ("swap in") another task.  The tasks under pre-emptive
        multitasking can be written as though they were the only task
        and the {scheduler} decides when to swap them.  The scheduler
        must ensure that when swapping tasks, sufficient state is
        saved and restored that tasks do not interfere.
     
        The length of time for which a process runs is known as its
        "{time slice}" and may depend on the task's priority or its
        use of resources such as memory and I/O.
     
        {OS/2}, {Unix} and the {Amiga} use pre-emptive multitasking.
     
        This contrasts with {cooperative multitasking} where each task
        must include calls to allow it to be {deschedule}d
        periodically.
     
        (1995-03-20)