Why is copper considered as a transition element even though it has completely filled d orbital i.e 3d10 configuration?

Although copper has 3d¹⁰ configuration,it can lose one electron from this arrangement.Hence,Cu⁺² has 3d⁹ configuration.So according to the definition that transition metal cations have partially filled (n-1)d subshell,copper can be regarded as a transition metal.