Compounds of structure $\ce{R2C(SR')OH}$ or $\ce{R2C(OR')SH}$ (monothiohemiacetals), or $\ce{R2C(SR')SH}$ (dithiohemiacetals), $\ce{R'} \neq \ce{H}$.
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]