Preemption
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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 displacing effect that federal law will have on a conflicting or inconsistent state law. The United States Constitution makes federal law the supreme law of the land. Thus, when there is a conflict between a state law and federal law, the federal laws trumps--or "preempts"--the state law. The term is also sometimes used to refer to the displacing effect state laws might have on ordinances enacted by municipalities.
Courts have developed an enormous and complicated body of caselaw to resolve conflicts between federal and state laws. As a general rule, there is a presumption in favor of the validity of a state law; thus, courts will attempt to reconcile seemingly inconsistent state and federal laws where possible. If the laws are truly irreconcilable, then the federal law will generally preempt the state law only to the extent of the inconsistency. There are many exceptions to these general rules, however. For example, Congress may declare its intent to make the federal government the primary source of law in a paricular area, which will result in state laws regulating that area being preempted even if they are not inconsistent with the federal law. Even in the absence of any indication that Congress indended to "occupy the field" of a particular subject matter, courts will be more likely to find a state law to be preempted by federal law if it touches upon an area where there has historically been a strong federal interest, such as banking or foreign affairs.
See also: Preemption Act of 1841