Domain Authority is a metric indicating a site's authority.

Domain Authority explained, a 3rd Party metric from Moz, seen by many in the SEO fraternity as the gold standard in determining a site’s perceived authority by Google and other search engines. The score is out of 100 and gets progressively harder the higher up the scale you go. As a rule of thumb a Domain Authority of 40+ is pretty respectful for most businesses with sites 60+ dominated by larger content publishers, charities and public bodies. Factors such as your site’s technical health and size and quality of your backlink profile are crucial in determining your domain authority. Check your site’s Domain Authority here at Open Site Explorer.