Coefficients (B, C, ...) in the virial equation of state for a real gas p V m = R T ( 1 + B V m + C V m where p is the pressure, V m the molar volume, R the gas constant and T the thermodynamic temperature.
Source:
Green Book, 2nd ed., p. 49 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
Purple Book, 1st ed., p. 58 (http://old.iupac.org/publications/books/author/metanomski.html)