Compounds in which the oxygen of a ketone has been replaced by divalent sulfur $\ce{R2C=S}$ ($\ce{R} \neq \ce{H}$), e.g. $\ce{CH3C(=S)CH2CH3}$ butane-2-thione.
Source: PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1372 [Terms] [Paper]