plenipotentiary UK /ˌplɛn.ɪ.pəʊˈtɛn.ʃəɹ.i/US /plɛn.ɪ.poʊˈtɛn.ʃ(i)əɹ.i/adjective 2noun 1forms 1 adjective2 •Invested with full power. •Of or relating to a plenipotentiary agent noun1 •A person invested with full powers, especially as the diplomatic agent of a sovereign state, (originally) charged with handling a certain matter.None but the like-minded can come plenipotentiary to our court. — 1859 formsplenipotentiaries