The hypothesis that, for given reactants, the reactions involving the smallest change in nuclear positions will have the lowest energy of activation. (It is also often simply referred to as principle of least motion.)
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1134 (https://doi.org/10.1351/pac199466051077)