An entity is something that has a distinct, separate existence, though it need not be a material existence. In particular, abstractions and legal fictions are usually regarded as entities. In general, there is also no presumption that an entity is animate. Entities are used in system developments as models that display communications and internal processing of, say, documents compared to order processing.An entity could be viewed as a set containing subsets. In philosophy, such sets are said to be abstract objects.The word entity is often useful when one wants to refer to something that could be a human being, a non-human animal, a non-thinking life-form such as a plant or fungus, a lifeless object, or even a belief; for instance. In this way, entity could be seen as a "catch all"-word.Sometimes, the word entity is used in a general sense of a being, whether or not the referent has material existence; e.g., is often referred to as an entity with no corporeal form, such as a language. T
Source:
http://en.wikipedia.org/wiki/Entity