† ˈtriarchistObs.rare—1. [f. tri- + Gr. ἀρχή beginning + -ist.] One who maintains the existence of three original principles of being. 1678Cudworth Intell.Syst. i. iv. §13. 216 Plutarch was both a Triarchist and a Ditheist,—an assertor of Three Principles, but of Two Gods.