Transformation of the general type
\[\ce{XABY -> A=B + XY (or X + Y, or X^{+} + Y^{-})}\] where the central atoms
\(\ce{A}\) and
\(\ce{B}\) are commonly, but not necessarily, carbon.
Source:
PAC, 2022, 94, 353. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)' on page 376 (https://doi.org/10.1515/pac-2018-1010)