Point of intersection between the potential energy surfaces of states of different multiplicity.
The intersection belongs to a (3N - 7)-dimensional subspace of the (3N - 6)-dimensional nuclear coordinate space and therefore appears as a line on a two-dimensional energy surface (N is the number of nuclei). In this case the @BT07335@ is one-dimensional and is defined by the @G02669@ difference vector x1.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 421 (https://doi.org/10.1351/pac200779030293)