Originally, the compound betaine, (CH3)3N+–CH2C(=O)ON,N,N-trimethylammonioacetate, and similar zwitterionic compounds derived from other amino acids. By extension, neutral molecules having charge-separated forms with an onium atom which bears no hydrogen atoms and that is not adjacent to the anionic atom. Betaines cannot be represented without formal charges. E.g.
See also: dipolar compounds, mesoionic compounds, ylides, zwitterionic compounds
PAC, 1995, 67, 1307 (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1322