This paper deals with new definitions of shielding effectiveness, in particular for high-frequency and transient electromagnetic fields. They are practicable and supposed to better characterize the shielding ability than the commonly used definitions. From the ratio of the time-averaged input power of the unshielded load to that one of the shielded load, in the limiting case of a vanishing load the electromagnetic shielding effectiveness is derived. This is a simple combination of the commonly used and easily measurable electric and magnetic shielding effectiveness. A similar procedure is then employed for the transient case, where in the limiting case of a vanishing load the ratio of the absorbed energies turn into the transient shielding effectiveness. Numerical results are shown for closed as well as for nonclosed cylindrical shields.