Coefficients of quantum numbers in the rotational term expression and inversely proportional to the principal moments of inertia. Symbols: A, B, C. A(tilde) = h8π2.c.IA (dimension wavenumber), A = h/8π^2.IA (dimension frequency) where h is the Planck constant and c the speed of light in vacuum.
Source:
Green Book, 2nd ed., p. 23 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)