A topic is a unit of information with a title and content, short enough to be specific to a single subject or answer a single question, but long enough to make sense on its own and be authored as a unit.
In DITA, a topic is the basic unit of authoring and of reuse. A document may contain one topic or multiple topics, and a document type may support authoring one or many kinds of topics. But regardless of where they occur, all topics have the same basic structure and capabilities. Books, PDF files, Websites, and help sets, for example, can all be constructed from the same set of underlying topic content, although there may be some topics that are unique to a particular deliverable, and the organization of topics may differ to take advantage of the unique capabilities of each delivery mechanism.
Reference information is inherently topic-oriented, since it requires information to be modular and self-contained for the sake of retrievability.
Topic-oriented authoring for conceptual and task information has its roots in Minimalism, an instructional design technique first espoused by John Carroll. The minimalist approach to information design focusses on identifying the smallest amount of instruction that allows for the successful completion of a task, or that provides basic knowledge of a concept. Readers have goals, and they want to achieve those goals as quickly as possible. Generally, readers don't want to read information just for the pleasure of reading. They are reading to learn or to do something.
While DITA's topic-oriented approach has its roots in instructional design, the topic-based approach can be useful for any information that has human readers and a consistent structure.
OASIS DITA Architectural Specification v1.0 -- 09 May 2005
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