Glossary /Authority & off-page
Backlink
A link from another site to yours, treated by engines as a third-party vote and still among the strongest ranking signals.
A backlink is an external site pointing at a page. The logic has not changed since PageRank: a link is an editorial decision to send readers somewhere, and enough of those decisions from credible sources is evidence a page is worth ranking.
Quality is not a spectrum so much as a cliff. A link from a publication in the field that genuinely covers you is worth more than a thousand from directories, aggregators and paid placements — and the thousand carry risk, since engines discount obvious link schemes and occasionally act against the site that built them.
The links that hold up are ones you could justify to the editor who placed them. That constraint rules out most of what the link-building industry sells and points at the things that actually work: original research, tools people reference, and being genuinely worth citing.