spin–spin coupling constant, $$J_{\text{AB}}$$

https://doi.org/10.1351/goldbook.S05876
coefficient of the indirect spin–spin coupling between two nuclei (A and B) in a magnetic resonance hamiltonian.
Source:
Green Book, 2nd ed., p. 15 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)