dispersion (for spectroscopic instruments)

Dispersion of a material = dn/dλ where n = refractive index and λ wavelength; angular dispersion = dΦ/dλ where Φ = angle; and linear dispersion = d x d λ where x = separation of spectral lines. The reciprocal of the last-named quantity is more frequently used (dλ/dx), and is commonly expressed in Å mm-1.
Orange Book, 2nd ed., p. 101 (http://media.iupac.org/publications/analytical_compendium/)