quaternary ammonium compounds

Derivatives of ammonium compounds, (NH4+)Y, in which all four of the hydrogens bonded to nitrogen have been replaced with hydrocarbyl groups. Compounds having a carbon-nitrogen double bond (i.e. R2C=N+R2Y) are more accurately called iminium compounds. e.g. [(CH3)4N]+OH, tetramethylammonium hydroxide.
See: onium compounds
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1361 (https://doi.org/10.1351/pac199567081307)