dispersion (for spectroscopic instruments)

https://doi.org/10.1351/goldbook.D01793
Dispersion of a material=dndλ where n =
refractive index
and λ
wavelength
; angular dispersion=dΦdλ where Φ =
angle
; and linear dispersion=dxdλ where x = separation of spectral lines. The reciprocal of the last-named quantity is more frequently used (dλdx), and is commonly expressed in  mm1.
Source:
Orange Book, 2nd ed., p. 101 [Terms] [Book]