Betaine is a neutral chemical compound
with a positively charged cationic functional
group such as a quaternary ammonium or
phosphonium cation (generally: onium ions)
which carries no hydrogen atom and has a
negatively charged functional group, such as a
carboxylate group, which may not be adjacent
to the cationic site. Hence a betaine may be a
specific type of zwitterion. Historically the
term was reserved for trimethylglycine only