silver(Ag) has a fully filled d orbital. then why is it called a transition element?

The term transition element, as defined by IUPAC, is the one with a partly filled d orbitals in its ground state or in any of its oxidation state. This definition justifies the inclusion of Cu, Ag and Au as transition metals, since Cu(II) has a 3d⁹ configuration, Ag(II) has a 4d⁹ and Au(III) has a 5d⁸ configuration. By this definition, Zn, Cd and Hg are excluded from transition metals, as they have d¹⁰ configuration. However, being the end members of the series, they are often considered together with the transition elements.