Rayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases.
Rayleigh scattering of sunlight in clear atmosphere is the main reason why the sky is blue: Rayleigh and cloud-mediated scattering contribute to diffuse light (direct light being sunrays).