In general, an entity (pronounced N-tih-tee ) is an existing or real thing. The word root is from the Latin, ens , or being, and makes a distinction between a thing's existence and its qualities. An entity exists and that's all it needs to do to be an entity. The fact that something exists also seems to connote separateness from other existences or entities. In programming, engineering, and probably many other contexts, the word is used to identify units, whether concrete things or abstract ideas, that have no ready name or label. In blackboard discussions, one can draw something as yet unnamed and refer to that drawing as the representation of an "entity." (If the entity being discussed later gets ascribed qualities and a name, reference to it as an "entity" may no longer be useful.)
Here are some of the usages we know of:
1) In the Standard Generalized Markup Language ( SGML ), an entity is a specific character string that has the effect of causing a formatting program (such as a print formatter or a Web browser - which formats for a display screen) to select and present a particular character or notation. In this usage, an entity is a certain string of characters that together specify a unique font to be selected and displayed or printed. A word was needed for such a character string and "entity" came in handy. In HTML (which is a usage of SGML), each special entity or character string is given a name and the entities are therefore called "named entities."
2) In relation to a database , an entity is a single person, place, or thing about which data can be stored.
3) In data modeling (a first step in the creation of a database), an entity is some unit of data that can be classified and have stated relationships to other entities.
4) In the Open Systems Interconnection ( OSI ) model of network communication, an entity is an active element within a subsystem that communicates with other entities using a defined protocol.
5) In IBM's RACF security product, an entity is a user, group, or resource that is defined to RACF.
6) In FORTRAN , almost every program element is referred to as an entity, such as a procedure, an operator, an interface block, an input-output unit, a symbolic constant, and a statement label.