The siemens, abbreviated S, is the SI unit of electrical conductance. It is the conductance which will allow a current of one ampere across a potential drop of one volt. The siemens is the reciprocal of the ohm.
Siemens and mho
Before the BIPM approved the siemens as a derived unit in 1971, the common unit for conductance was the mho (ohm spelled backwards), abbreviated ℧. The mho is still in use, despite not having official status, as the upside-down omega is not likely to be confused with lower-case s for seconds, nor for S occurring in formulas.