Backlink

A backlink is a reference in the reverse direction: given a page, the set of pages that link to it. Forward links are authored; backlinks are computed.

That asymmetry is what makes them valuable. Writing a link requires deciding, at that moment, that a connection matters. Backlinks accumulate without anyone deciding anything, and reading them later surfaces the contexts a concept turned out to belong to — including ones the author of the target page never anticipated.

They also change what a stub is worth. In a system with backlinks, a link to a page that does not exist yet is not an error but a signal: the emerging list of references to a not-yet-written page is a fairly good measure of whether it is worth writing.

Backlinks scale in a way that manual cross-referencing does not. Maintaining "see also" sections by hand means every new note requires editing every related note; with backlinks, linking outward once produces the inbound reference automatically.

The same idea, at web scale, was the basis of link-based search ranking: a page's inbound links as a measure of its significance. That worked until it became worth manufacturing, which is the general fate of any measure turned into a target. See Goodhart's Law.