Siemens is the official SI unit measuring the electric conductance of a material. For all practical purposes, conductance is the reciprocal of resistance, that is:
Conductance = 1/Resistance
The siemens (abbreviated S; not to be confused with the lower-case s for seconds), can be expressed in SI base units as:
S = m-2·kg-1·s3·A2
Of course, realizing that the siemens is the reciprocal of the ohm would give you the much simpler formula derived from Ohm's Law:
S = A/V
It is interesting to point out that the siemens used to be known as the mho (ohm spelled backwards) up until 1971, when the new name was adopted at the 14th Conférence Générale des Poids et Mesures, in the middle of a move to rename units after dead, important people in the field (in this case, Werner von Siemens).
Many of you who have casually taken physics or maybe even entry-level electrical engineering may not have heard of this unit. It really isn't one of the more important ones (it IS a reciprocal, after all). Plus, no teacher would want to be caught mouthing "That's a lot of siemens!" or "40 megasiemens!" Right.