This paper puts forward the basic principle for definition of apparentresistivity in electrical prospecting, and on such a basis discusses merits anddemerits of various sorts of apparent resistivity definition in MT, TEMS andFEMS methods with some suggestions for improvement made. Studies showthat in MT method the apparent resistivity defined by using real part ofplane impedance or given in time domain is very effective, which can wellrecognize the good conductive layer, while the apparent resistivity defined byusing imaginary part of plane impedance yields the worst result. So far asTEMS method is concerned, the whole-region definition of apparent resistivityis more effective than distant-region or nearby-region approximate definition.Studies also reveal that the whole-region apparent resistivity defined by timederivative of magnetic field sometimes has multi-solutions or even nosolution,whereas the apparent resistivity defined by magnetic field itseif is quiteeffective. For FEMS method, similar discussion is also made concerning thedefinition of whole-region apparent resistivity, and the result obtained, thoughnot so ideal as the result of TEMS method, is somewhat better than the appa-rent resistivity response defined under the condition of commonly-used largeand small induction values. This paper has made physical interpretation of thefalse extremum effect of apparent resistivity response curve that appears inelectromagnetic sounding method.