Backlink
A backlink is a link from one website to another, and it acts as a signal that search engines use to judge a page's authority and relevance.
When another site links to one of your pages, that is a backlink (also called an inbound link). Google treats each one as a kind of vote: the more high-quality, relevant sites that link to you, the more trustworthy your page tends to look. This is the core idea behind PageRank and why link building exists as a discipline.
Not all backlinks carry the same weight. A contextual link placed inside the body text of a topically relevant, authoritative page passes far more value than a footer link or a comment on an unrelated blog. The link attributes matter too: a dofollow link can pass ranking signals, while a nofollow link generally does not, though Google now treats nofollow as a hint rather than a strict rule.
Quality beats quantity. A handful of links from respected sites in your niche will usually outperform hundreds of low-value ones, and aggressively chasing cheap or manipulative links (think private blog network or paid link scheme networks) can trigger a manual action rather than a boost. Focus on links you would be proud to show a search engineer.