Backlink
A backlink is a hyperlink on one site pointing to a page on another. Search engines use backlinks both to discover URLs and as an indication that other people found a page worth referencing, which is why link signals have remained part of ranking since the beginning.
Backlinks do two separate jobs, and conflating them causes confusion.
Discovery. A link is how a crawler finds a URL it did not previously know about. This works regardless of the link’s quality.
Endorsement. A link is a weak signal that someone thought the target was worth pointing at. The original PageRank paper formalised this: a page linked from many pages is likely more important, and a link from an important page counts for more.
What has changed since then is how much filtering sits on top. Links that are
paid for, exchanged, placed at scale, or clearly not editorial are discounted or
ignored, and rel="sponsored", rel="ugc" and rel="nofollow" exist to
declare a link’s nature.
The measurable truth is narrower than the industry’s confidence about it: link counts correlate with rankings, and correlation is not the same as a formula you can exploit. A link from a relevant page that a real person placed because the content deserved it is the only version of this that reliably holds up.