Compounds having the structure $\ce{R2S=O}$ ($\ce{R} \neq \ce{H}$), e.g. $\ce{Ph2S=O}$ diphenyl sulfoxide.
Source: PAC, 1995,
67, 1307. (
Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1370 [
Terms] [
Paper]