Mechanics Turbulence is fluid flow in which a random agitation is superposed over the mean movement. Turbulence is characterised by matter being transferred in directions different from the mean flow direction (diffusion).
In a turbulent flow the velocity at any given instant can vary widely and unpredictably for points that are close to each other. Turbulence is a viscosity related phenomenon the study of which is very complex, involving concepts and methods from statistical physics (see. mean free path) or dynamic systems.
Meteorology Turbulence occurs in the atmosphere, especially in wind. Turbulence is one of the factors that makes it difficult to forecast weather more than five days ahead.
Astronomy Turbulence affects the quality of images obtained using a visual or photographic observation instrument. This turbulence is mainly due to thermal phenomena (heat waves) or dynamic phenomena (dust movements) in the atmosphere.
link to lectures on turbulence.