Illustrated Glossary of Organic Chemistry

Quaternary ammonium salt (4o ammonium salt): An ammonium salt in which the nitrogen atom bearing the +1 formal charge has four carbon-nitrogen single bonds (i.e., it is bonded to four carbon groups).

Methylammonium chloride
A primary (1o) ammonium salt
Dimethylammonium chloride
A secondary (2o) ammonium salt
Trimethylammonium chloride
A tertiary (3o) ammonium salt
Tetramethylammonium chloride
A quaternary (4o) ammonium salt
1,1-diphenylpyrrolidinium bromide
A quaternary (4o) ammonium salt