1. The transport of matter solely by the random motions of individual molecules not moving together in coherent groups. Diffusion is a consequence of concentration gradients. 2. The process of mixing fluid properties by molecular and turbulent motions. The process in turbulent flow is called turbulent or eddy diffusion. Diffusion by turbulence is much more rapid than diffusion by molecular motions. Turbulence with statistics independent of direction is called isotropic. In general, turbulence in the atmosphere and ocean is not isotropic. See diffusivity.