tetrahedral intermediate

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).
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1173 (https://doi.org/10.1351/pac199466051077)