Point of intersection between the potential energy surfaces of states of different
multiplicity.

Note:

The intersection belongs to a
()-dimensional
subspace of the
()-dimensional nuclear coordinate space and therefore appears
as a line on a two-dimensional energy surface
(
is the number of nuclei).
In this case the branching plane is one-dimensional and is defined by the gradient
difference vector
.

Source:

PAC, 2007, *79*, 293*
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
* on page 421