Empirical SEOs are Digital Marketers for organic search like Google that question what's going on, try to understand the algorithms, filters and changes by observation, testing, and experimentation.
This is what SEO was original, before the strong influence of Google and the "Penguin Shock".
Hat tip to Ross Tavendale who coined this phrase in January 2019.
Empirical SEO, therefore, refers to ongoing testing and validating ideas, by potentially risking an unexpected reaction from the search engine, to revert it in an additional step and learn for future optimization. This is required because there is no "SEO playbook" and certainly no rule set given by Google and other search engines how their algorithms could be exploited for the benefit over competitors.