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 [Terms] [Paper]