Two (or more) molecules with the same spin multiplicity but with different lengths for one or more bonds.
Note:
This feature arises because the potential-energy surface, which describes how the energy of the molecule depends on geometry, shows two (or more) minima that are not merely symmetry-related.
Source:
PAC, 2022, 94, 353. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)' on page 373 (https://doi.org/10.1515/pac-2018-1010)