1. The random movement of molecules or ions or small particles in solution or suspension under the influence of brownian (thermal) motion toward a uniform distribution throughout the available volume; the rate is relatively rapid among liquids and gases, but takes place very slowly among solids. 2. Light scattering.
- facilitated d. See facilitated transport.
- gel d. d. in a gel, as in the case of gel d. precipitin tests in which the immune reactants diffuse in agar. SEE ALSO: immunodiffusion.
- passive d. See facilitated transport.

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dif·fu·sion dif-'yü-zhən n
1) the process whereby particles of liquids, gases, or solids intermingle as the result of their spontaneous movement caused by thermal agitation and in dissolved substances move from a region of higher to one of lower concentration
2 a) reflection of light by a rough reflecting surface
b) transmission of light through a translucent material
dif·fu·sion·al -'yüzh-nəl, -ən-əl adj

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dif·fu·sion (dĭ-fuґzhən) 1. the process of becoming diffused, or widely spread. 2. the spontaneous movement of molecules or other particles in solution, owing to their random thermal motion, to reach a uniform concentration throughout the solvent, a process requiring no addition of energy to the system. 3. the movement of solutes down concentration gradients across a semipermeable membrane such as a hemodialyzer membrane; the movement is generally of urea and low molecular weight toxins from the blood to the dialysate and of bicarbonate and acetate from the dialysate to the blood. 4. immunodiffusion.

Medical dictionary. 2011.