The scattering of electromagnetic radiation by spherical particles of any size r, relative to the wavelength, λ. Since the cases r ≪ λ and r ≫ λ are covered by Rayleigh (dipole) scattering and geometric scattering theories, respectively, Mie scattering often refers to the case of r ≈ λ.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2202 (https://doi.org/10.1351/pac199062112167)