Preemption as used with respect to operating systems means the ability of the operating system to preempt or stop a currently scheduled task in favour of a higher priority task. The scheduling may be one of, but not limited to, process scheduling, I/O scheduling etc.

Non-preemptibility arises, for instance, when handling an interrupt. In this case, scheduling is avoided until the interrupt is handled. Making a scheduler preemptible has the advantage of better system responsiveness and scalability.

Most modern operating systems, such as various flavours of Unix, the scheduler can preempt user process. Some operating systems' schedulers have the ability to preempt system calls (kernel calls) as well.

In the American legal system, preemption generally refers to the Federal Government of the United States' power to regulate certain areas of activity, such as interstate telephone service, exclusively, preventing state or local governments from also regulating the area. State governments, however, may sometimes preempt municipalities from regulating in certain areas.