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Derrick, "Domain authority" is pretty much a term that Moz coined back around 1995 as an alternative to Google's term "pagerank". Essentially the two describe the same thing and Moz's tools evolved since then as a means to quantify the factors that make up pagerank/authority.  Moz's  tools are a good place to start wrapping your arms around the strength of a competitor's site as compared to yours.