Transclusion

Transclusion is the inclusion of content from one document into another by reference, rather than by copying the content. The including document contains a link or directive to the source, and at render time the system retrieves and embeds the source content. This allows a single source to appear in multiple places and to be updated in one place when the source changes.

The term was coined by Ted Nelson in the 1960s as part of the Xanadu project, as a mechanism to create a network of linked documents with persistent identity, enabling content to be reused without duplication. Transclusion is contrasted with transcriptions or copies.

In practice, transclusion can be implemented in various ways: in HTML via iframes or dynamic content loading; in wikis via template transclusion (MediaWiki uses templates included in pages); in software build systems via preprocessor #include; in digital publishing via Xanadu-like systems.

Transclusion can be partial, including only certain sections, and it can be parameterized. It preserves the source's identity, so updates propagate to the including document when re-rendered, subject to caching and access controls.

Pros and cons: advantages include reduced duplication, improved consistency, and centralized updates. Drawbacks include introduced dependencies, potential performance costs, complexity in rendering order, risk of circular or recursive inclusion, and considerations around licensing and attribution.

Applications include hypertext systems, wikis, documentation, content management, and certain programming environments. The idea remains influential in discussions of modular, reusable content and the design of systems that separate content from its presentation.