Transclusion: the same content, knowably, in more than one place
Nelson's definition is the same content knowably in more than one place. When a passage is transcluded into a new document it is not duplicated; it is shown there, live, by reference, and the reader can follow it back to where it came from. Change the original and every transclusion changes. Quote a paragraph and the quotation stays connected to its context. The word appears in Literary Machines (1981), and Nelson says the idea was already in his 1965 paper.
This is the heart of the Xanadu copyright proposal he called transcopyright. If a quotation is a window onto the original rather than a copy of it, the original author can be paid a small amount each time the passage is read, wherever it is read, and nobody needs permission to quote. The reader sees the whole, the author keeps the rights, and the accountancy is automatic. The seventeen rules put it plainly: every document can contain a royalty mechanism for payment on any portion accessed, including transclusions.
The Web does the opposite. A quotation on a web page is a paste; it has no memory of where it came from, and the source can change or vanish without the quotation knowing. Embedded images are the nearest the Web gets to transclusion, and they show both the power and the fragility of the idea when the link goes only one way. Wikipedia's article on the subject distinguishes transclusion (stored once, reused everywhere, updated everywhere) from substitution (a static copy that must be managed by hand).
Nelson also demonstrated Web transclusion with the Little Transquoter, programmed by Andrew Pam in 2004-05, which makes references to portions of web pages that remain click-connected to their original contexts. It is a small proof that the idea works even on the platform built without it.
Sources: Wikipedia, Transclusion; Wikipedia, Project Xanadu; Wikipedia, Literary Machines. Where this page paraphrases him, the paraphrase is ours and the source is his.
More ideas
Hypertext: writing that branches
The word is his, from 1963. So is the idea that a text need not be a line.
Project Xanadu: the original hypertext project
Founded 1960. Still not finished, and still the most complete alternative to the Web anyone has designed.
Intertwingularity: there are no subjects
A 1974 portmanteau for the fact that knowledge does not come in boxes.
The docuverse and tumblers: one library, every version
The universe of documents, and an address for every character in it.
ZigZag: a spreadsheet with as many dimensions as you need
Conceived at Datapoint in 1981; patented in 2001; built by a small team ever since.