shielding constant, $$\sigma$$

https://doi.org/10.1351/goldbook.S05645
In NMR the difference between the external magnetic flux density and the local magnetic flux density at a resonating nucleus affected by the neighbouring electrons divided by the external flux density, σ = B 0- B B 0.
Source:
Green Book, 2nd ed., p. 25 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)