The Collaborative International Dictionary of English v.0.48:

Diffusion \Dif*fu"sion\, n. [L. diffusio: cf. F. diffusion.]
   1. The act of diffusing, or the state of being diffused; a
      spreading; extension; dissemination; circulation;
      dispersion.
      [1913 Webster]

            A diffusion of knowledge which has undermined
            superstition.                         --Burke.
      [1913 Webster]

   2. (Physiol.) The act of passing by osmosis through animal
      membranes, as in the distribution of poisons, gases, etc.,
      through the body. Unlike absorption, diffusion may go on
      after death, that is, after the blood ceases to circulate.

   Syn: Extension; spread; propagation; circulation; expansion;
        dispersion.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diffusion
    n 1: (physics) the process in which there is movement of a
         substance from an area of high concentration of that
         substance to an area of lower concentration
    2: the spread of social institutions (and myths and skills) from
       one society to another
    3: the property of being diffused or dispersed [syn:
       dissemination, diffusion]
    4: the act of dispersing or diffusing something; "the dispersion
       of the troops"; "the diffusion of knowledge" [syn:
       dispersion, dispersal, dissemination, diffusion]