Species of the form
RO+ , e.g.
CH3O+ methoxylium,
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 (https://doi.org/10.1351/pac199567081307)