1. The reciprocal of impedance (q.v.); the complex ratio of current to voltage. The unit of measure is the siemens or mho. 2. The admittance of a section consisting of horizontal layers of thickness h and resistivity ρ overlaying a basement of very high resistivity is S=Σhi/ρi. See also S-rule. 3. In the magnetotelluric method, the complex ratio of electric field E to the perpendicular magnetic field H, or the inverse of the impedance tensor Z.