oxylium ions

https://doi.org/10.1351/goldbook.O04382
Species of the form $\ce{RO^{+}}$, e.g. $\ce{CH3O^{+}}$ methoxylium, $\ce{HO^{+}}$ hydroxylium.
Source:
PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1355 [Terms] [Paper]