A saddle point on a potential-energy (reaction) surface. It has one negative force constant in the harmonic force constant matrix.
See also: activated complex, transition state
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1174 (https://doi.org/10.1351/pac199466051077)