Domain Authority (DA, Moz), Domain Rating (DR, Ahrefs), Domain Score, various names, similar idea: a score from 0-100 estimating how strong a site's backlink profile is. Useful proxy; not a Google metric.
Google doesn't publish PageRank anymore. All the "Authority" scores you see are third-party estimates based on public link data. They correlate with ranking ability but aren't what Google uses internally.
Each tool uses its own crawl and algorithm. A site can show DA 50 in Moz and DR 60 in Ahrefs, neither is "right."
Same methodology applied to individual URLs. A page's PA/UR is usually lower than the domain's overall score, except for homepages.
At a high level: number of referring domains, quality of those domains (their authority scores), link placement, anchor text diversity. Each tool weights these differently.
Check your DR vs competitors'. If you're DR 30 and top competitor is DR 60, you know you need serious link-building to rank competitively for hard queries.
KD of 40 is trivial if your DR is 70; a wall if your DR is 15. Adjust keyword targeting.
Deciding whether to pursue a guest post on site X? Their DR is a quick (imperfect) proxy for the link's value.
Watch your DR/DA over 6-12 months. Rising = link acquisition working. Flat or falling = momentum problem.
A DR 70 site that built authority through gray-hat links is less trustworthy than a DR 40 site with all organic links. Score doesn't distinguish.
A general-purpose DR 80 site might rank lower for a niche topic than a DR 40 site with deep topical authority in that niche. Score is topic-agnostic.
Some services inflate DA/DR by building links just for the score. Always check underlying referring domains and organic traffic, not just the summary number.
A 15-year-old DR 50 site is usually more valuable than a 1-year-old DR 50 site (age is a proxy for sustained trust).
Rather than obsessing over DA/DR, watch your organic traffic curve. That's the only score that matters for revenue. DR should correlate, but it's a lagging indicator.
Authority scores are relative, not absolute. Within one tool, comparing your DR to competitors' DR is useful. Across tools, scores don't translate.