A reaction
intermediate in which the bond arrangement around an initially double-bonded carbon atom (typically
a carbonyl carbon atom) has been transformed from trigonal to tetrahedral. For example,
aldol in the
condensation reaction of acetaldehyde (but most tetrahedral intermediates have a more fleeting existence).
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1173