One of the (arguably the) original hypertext visions (AFAIK Ted Nelson, its creator, coined the term "hypertext" in the early 60s).
The main difference to web hypertext as we know it is that it doesn't just link documents together by what we know as "links", but it's bidirectional and with more content inclusion. E.g. if someone takes content from another document and puts it in their article, the text is connected to the source document (instead of them maybe putting a link to the source). And from the source document you can find all documents that used parts of it.
Project Xanadu is Nelson's repeated attempts to make this work, while the world choose the WWW model and is happy enough with that (much to the disdain of Xanadu supporters, who believe the WWW to be a vastly inferior and broken approach). Once HTML took off, there wasn't really a chance to catch up, especially not with the limited resources they have. Lot's of bitterness about that, which also means they haven't really tried to make their ideas work with the web and rather aiming for an uncompromising, pure implementation of their ideas.
The main difference to web hypertext as we know it is that it doesn't just link documents together by what we know as "links", but it's bidirectional and with more content inclusion. E.g. if someone takes content from another document and puts it in their article, the text is connected to the source document (instead of them maybe putting a link to the source). And from the source document you can find all documents that used parts of it.
Project Xanadu is Nelson's repeated attempts to make this work, while the world choose the WWW model and is happy enough with that (much to the disdain of Xanadu supporters, who believe the WWW to be a vastly inferior and broken approach). Once HTML took off, there wasn't really a chance to catch up, especially not with the limited resources they have. Lot's of bitterness about that, which also means they haven't really tried to make their ideas work with the web and rather aiming for an uncompromising, pure implementation of their ideas.