The Free On-line Dictionary of Computing (18 March 2015):
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 descheduled