The cation produced by hydronation of a solvent molecule. For example,
(CH3OH2+) is the lyonium ion of methanol.
See also: onium compounds
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1137 (https://doi.org/10.1351/pac199466051077)