By the term parallelism we shall understand that property of an entity by virtue of which it is capable of several distinct actions at the same time. This may be the case if the entity is composed of spatially separated sub-entities which co-ordinate their activities in some way, but are capable of autonomous action. In such cases, it is natural to speak of such entities as distributed. Of course, distributed entities usually possess parallelism, although one does not always conceive of an entity possessing parallelism as being distributed. I may be able to walk and chew gum at the same time, but I do not consider myself a distributed system (although an anatomist might). Other terms used for such entities are asynchronous and concurrent; we offer formal interpretations of ‘asynchronous’ and ‘concurrent’ in chapter 2.
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