A link from a site outside of your site.
Inbound links send visitors to your web site. Generally, this is seen as a good thing. Many sites go to great lengths to achieve as much of this “free” advertising as possible, although a few sites are very particular about where the links are pointing (see deep linking).
see also: SEO Tools
SEMrush vs. Ahrefs vs. Moz (includes backlink comparison)
MarketingTerms.com (April 20, 2019)