Extended definition
Number of unique domains linking to your site. More important than the total backlink count, because it reflects diversity.
Context and application
Referring domains is a trust indicator — Google prefers natural diversity. A site receiving 1000 backlinks from 5 domains is suspicious (link farm? massive sponsorship?). A site receiving 200 backlinks from 150 domains seems naturally earned. Practical strategy: focus on diversified outreach (10 different sectoral sites > 1 site with 10 links). Tools like Ahrefs and SEMrush report referring domains in Site Explorer; for own tracking, export monthly and calculate growth rate.
In practice
100 backlinks from 100 different domains > 1,000 backlinks from 10 domains.