MOZ has a great article about domain authority: Domain Authority
Domain Authority represents Moz's best prediction for how a website will perform in search engine rankings. Use Domain Authority when comparing one site to another or tracking the “strength” of your website over time. We calculate this metric by combining all of our other link metrics—linking root domains, number of total links, MozRank, MozTrust, etc.—into a single score.
To determine Domain Authority, we employ machine learning against Google's algorithm to best model how search engine results are generated. Over 40 signals are included in this calculation. This means your website's Domain Authority score will often fluxuate. For this reason, it's best to use Domain Authority as a competitive metric against other sites as opposed to a historic measure of your internal SEO efforts.