Backlink
Simple Definition
A link from another website pointing to your website.
Technical Definition
A backlink (also called an inbound link or external link) is a hyperlink on one domain that points to a page on a different domain. Backlinks are a core Google ranking signal. High-quality backlinks from authoritative, relevant websites increase a page's PageRank and improve its likelihood of ranking well.
Example
If the New York Times publishes an article and links to your website, that is a backlink. It signals to Google that your content is worth referencing.