Acids in which an acidic $\ce{–OH}$ group has been replaced by an $\ce{–OOH}$ group, e.g. $\ce{CH3C(=O)OOH}$ peroxyacetic acid, $\ce{PhS(=O)2OOH}$ benzeneperoxysulfonic acid.
Source: PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1356 [Terms] [Paper]