From AMS Glossary
The mean cosine of the scattering angle, found by integration over the complete scattering phase function.
The asymmetry factor is a fundamental property of a scattering medium that affects the transfer of radiant energy through the medium. Particles that are small compared to the wavelength of the radiation, such as air molecules, have asymmetry factors close to zero. Larger particles, such as cloud droplets, typically have asymmetry factors ∼0.85 for visible radiation, consistent with strong forward scattering.