Transclusion is the inclusion of the same content in more than one place, so a passage shown inside a second document remains one item tied to its source; block references and block embeds apply the idea to single paragraphs in note-taking tools.
📝Ted Nelson uses the term in his writing for Project Xanadu, where he describes transclusions as neither copies nor instances but the same content appearing, knowably and visibly, in more than one place. In Nelson's account it is the visible re-use of content, working alongside links between documents, the subject of the sibling entry 📝Bidirectional Links / Backlinks. Xanadu's transcopyright license carries the idea into publishing: a republisher sends the address of content rather than a copy, so every portion stays attached to its original source.
Note-taking tools apply transclusion at the level of the block. Obsidian's documentation defines a block as a unit of text such as a paragraph, block quote or list item; a block given a caret-marked identifier can be linked from another note or embedded in it, displaying inside the host note. Logseq's documentation describes a block reference as a clickable link to a particular block, written as the block's unique identifier inside double parentheses.
Block references and embeds are a feature of the 📝Networked Thought tools, including 📝Obsidian and 📝Logseq, and a working method in 📝Personal Knowledge Management: one passage can serve several contexts without being duplicated.