One one-thousandth of a siemens. From Wikipedia: "The siemens is the SI derived unit of electric conductance. It is named after the German inventor and industrialist Ernst Werner von Siemens, and is equivalent to the previously used designation of this unit, the mho. In English, the term siemens is used both for the singular and plural. The 14th General Conference on Weights and Measures approved the addition of the siemens as an SI derived unit in 1971."