The impedance or admittance of an alternating-current circuit. It is sometimes convenient to use the term immittance when referring to a complex number which may be either the impedance (ratio of voltage to current) or the admittance (ratio of current to voltage) of an electrical circuit. The units of impedance and admittance are, of course, different and so units cannot be assigned to an immittance. However, in certain theoretical work it may be necessary to deal with general functions which afterward will be specialized to become either an impedance or an admittance by the assignment of suitable units; in such cases it is convenient to refer to the functions as immittances.