Definition of 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+R2 Y-) are more accurately called iminium compounds. See under onium compounds. E.g. [(CH3)4N]+ OH- tetramethylammonium hydroxide