sulphonium

from The Century Dictionary.

• noun A hypothetical univalent radical,—SH₃. Derivatives in which the hydrogen is replaced by hydrocarbon radicals, such as methyl, CH₃, are known and resemble the corresponding ammonium compounds in constitution and general properties.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) A hypothetical radical, SH3, regarded as the type and nucleus of the sulphines.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Alternative spelling of sulfonium.