SGML entity

In the Standard Generalized Markup Language (SGML), an entity is a primitive data type, which associates a string with either a unique alias (such as a user-specified name) or an SGML reserved word (such as #DEFAULT). Entities are foundational to the organizational structure and definition of SGML documents. The SGML specification defines numerous entity types, which are distinguished by keyword qualifiers and context. An entity string value may variously consist of plain text, SGML tags, and/or references to previously-defined entities. Certain entity types may also invoke external documents. Entities are called by reference.

Entities are classified as general or parameter:

Entities are also further classified as parsed or unparsed:

An internal entity has a value that is either a literal string, or a parsed string comprising markup and entities defined in the same document (such as a Document Type Declaration or subdocument). In contrast, an external entity has a declaration that invokes an external document, thereby necessitating the intervention of an entity manager to resolve the external document reference.

An entity declaration may have a literal value, or may have some combination of an optional SYSTEM identifier, which allows SGML parsers to process an entity's string referent as a resource identifier, and an optional PUBLIC identifier, which identifies the entity independent of any particular representation. In XML, a subset of SGML, an entity declaration may not have a PUBLIC identifier without a SYSTEM identifier.

When an external entity references a complete SGML document, it is known in the calling document as an SGML document entity. An SGML document is a text document with SGML markup defined in an SGML prologue (i.e., the DTD and subdocuments). A complete SGML document comprises not only the document instance itself, but also the prologue and, optionally, the SGML declaration (which defines the document's markup syntax and declares the character encoding).

...
Wikipedia