dispersion (for spectroscopic instruments)

Dispersion of a material = d n d λ where n = refractive index and λwavelength; angular dispersion = d Φ d λ 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 ( d λ d x), and is commonly expressed in Å mm −1.
Orange Book, p. 101